# How long will it be until the nearby groups of galaxies are receding at speeds faster than light?

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And subsequently how long will it be until we can never see them again?

This will take about 2 trillion years.

Given everything we know now, it's not too hard to predict the eventual fate of the universe. Of course there may be some changes as our knowledge advances, but I think the general course of events will occur as we expect them to. Based on our current knowledge, if you run through the math, you find that after about 2 trillion years from now, the expansion of the universe will have made it so that all by the closest galaxies will no longer be visible to us. You do ask two separate questions, but they're more or less the same question. They might not occur precisely at the same time, but predictions of the future aren't precise enough to distinguish such a small time difference.

By the way, you can read more about the future of the universe on Wikipedia, and specifically this section as it relates to your question.

## Surface of Last Scattering

When we look at the Andromeda galaxy, we are seeing photons that are 2 million years old (that is how long it took the photons, traveling at the speed of light, to reach us since the distance between our galaxy and the Andromeda galaxy is 2 million light-years) since the scaling factor (a(t)) a t was smaller then, the photons were “hotter” and had more energy. If we look back at the light coming from galaxies 10 billion light-years away, we are seeing photons that are 10 billion years old and that are even hotter and more energetic. If we look back about 13.4 billion light-years, we are seeing photons that are 13.4 billion years old and that were buzzing along through space when the Universe was just 300,000 years old we are seeing photons right at the time when the Universe transitioned from being opaque to being transparent. Since these photons are 3000K, when we look up at the night sky it should be blindingly bright. Indeed, if we ignore the expansion of space due to (ρ) ρ , this would be the case. However, because these photons are traveling through a space that is expanding, they become redshifted to the microwave range.

3K). During some time period of the very young Universe, the temperature was so hot that light and photons couldn’t pass through it (they became scattered). The hydrogen and helium atoms were ionized this means that the electrons and nuclei (or protons and neutrons were split too, not sure) were split apart and buzzing passed one another at tremendous speeds. Such a state of matter is called a plasma. During this time period, all of the matter in the Universe looked like a giant, glowing ball of plasma (in other words, it looked like a giant Sun). When the temperature of a collection of particles become (

3000 K , it becomes opaque (which means that light and photons cannot pass through it). We will call this temperature (T_< ext>) T last scattering .

The temperature (T_< ext>≈3,000K) T last scattering ≈3000 K is the “turning point” so to speak at the time (which we will call (t_< ext>) t last scattering ) when atoms and radiation were at this temperature, this is the instant of time just before the Universe became transparent. For any (t>t_< ext>) t > t last scattering , the temperature (T) T of radiation and atoms was (T<T_< ext>≈3,000K) T < T last scattering ≈3000 K . We are interested in solving for the time (t_< ext>) t last scattering : that moment of time when the Universe transitioned from being opaque to transparent. To do this, we take advantage of the fact that the Universe was still matter dominated at (t_< ext>) t last scattering assuming that throughout the entire time interval from (t_< ext>) t last scattering all the way until our present time (t) t the dominant form of energy density in the Universe was (ρ_M), the scaling factor is (a(t)=Ct^<2/3>) a t = C t 2/3 for this entire interval of time. (It is an enormous idealizations and simplification to assume the only contribution to the energy density (ρ) ρ throughout the Universe is (ρ_M) ρ M and that (ρ≈ρ_M) ρ ≈ ρ M later on, we shall come up with a model from which we can derive a (ρ) ρ that includes both radiation energy density (ρ_r) ρ r and mass-energy density (ρ_M) ρ M .) At our present time (which we will call (t_≈10^<10> ext< years>=10 ext< billion years>) t today ≈ 10 10 years =10 billion years ), the scaling factor is given by (a(t_)=Ct^<2/3>_) a t today = C t today 2/3 at the time (t_< ext>) t last scattering , the scaling factor was (a(t_< ext>=Ct^<2/3>_< ext>). By taking the ratio of the two scaling factors and doing some algebra, we can find (t_< ext>) t last scattering :

a t today a t last scattering = C t today 2/3 C t last scattering 2/3 =1000 egin
frac<>)><>>)>=frac_>_< ext>>=1000.
end Since (T≈1/a) T ∝ 1 a , if we are considering a time when (T) was 1,000 times greater than it is today, the scaling factor was 1,000 times smaller back at that hotter (T)

egin
frac<>><>>>=1000^<3/2>⇒t_< ext>≈frac<10^<10>years><1000^<3/2>>≈300,000 ext< years>.
end For the first 300,000 years of the life of the Universe, the Universe was opaque. Just after a time of roughly 300,000 years since the Big Bang, the temperature (T) T of the Universe became cool enough for atomic nuclei to bond with electrons and form electrically neutral hydrogen and helium atoms allowing the Universe to become transparent.

The graph of (ρ(a)) vs. (a) shows that as (a) a increases (since (k=0), (a) a will keep growing with (t) t forever) with time and the Universe expands, during the time interval (t_0≤t≤t_1) 0≤ t ≤ t 0 when the scaling factor increased from (a(t_0)) a (0) to (a(t_1)), the dominant form of energy density was (ρ_r) ρ r since (ρ_r>ρ_M) ρ r > ρ M and (ρ_r>ρ_0) ρ r > ρ 0 over this time period. During the time interval (t_0≤t≤t_1) t 0 ≤ t ≤ t 1 when the scaling factor increased from (a(t_0)) a ( t 0 ) to (a(t_1)), the dominant form of energy density was (ρ_M) ρ M . For times (t>t_1) t > t 1 when (a(t)>a(t_1)) a ( t )> a ( t 1 ) (which is the time period we live in today), the dominant form of energy density is (ρ_0) ρ 0 . From the graph for (a(t)) a ( t ) vs. (t), we see that after a long enough time (when (t) t becomes very large), (a(t)=Ce^<3>πGρ_0>t>) a t = C e 8 3 πG ρ 0 t will dominate and the Universe will keep expanding at an exponential (and thus accelerating) rate.

How (a(t)) a ( t ) changes with (t) t in a Universe dominated only by dark/vacuum energy only becomes significant over vast distance scales. Substituting (a(t)) a ( t ) into (D=a(t)∆r) D = a ( t )∆ r and then substituting (D) D into (V=H_0D), we can calculate that the recessional velocities between our galaxy and very nearby galaxies (small D ) is very small (it is only hundreds of kilometers per second which, compared to the speed of light, is extremely small). For very enormous values of (D) D , the recessional velocities of distant galaxies is close to the speed of light. For example, distant quasars that are 10 billion light-years away move away from as at a speed of about half the speed of light. If the distance (D) D a galaxy is away from us is big enough, it will be moving away from us at a speed greater than the speed of light. As the march of time progresses, Hubble's Parameter (H(t)) will continue to grow with time since the scaling factor will continue to increase with time due to dark energy after an unimaginably long period of time (five billion years), (H(t)) will have increased so dramatically that even if one substituted distances for the nearest galaxies only a few million light-years away into Equation (3), they would discover that those galaxies are receding from us faster than light speed. Lawrence Krauss once said that, for this reason, we are living in a very special time in the history of the universe, a time when we can still observe the CMBR and arrive at correct conclusions about the nature of the universe. Five billion years from now, all of the galaxies beyond the Milky Way, and also the CMBR, will be receding from us faster than the speed of light and will become undetectable.

1. Singh, Simon. Big Bang: The Origin of the Universe. New York: Harper Perennial, 2004. Print.

## How do we know that the universe is constantly expanding?

The velocity of a galaxy along our line of sight is fairly easy to determine, since we can use the Doppler shift of atomic spectral lines to measure that extremely accurately. (It’s much, much more difficult to measure their velocity perpendicular to our line of sight. So much so, that this is currently only possible for the most nearby galaxies caught in the grip of local gravitational interactions, not those with motions dominated by the universe’s expansion.)

What this means is that we can get a very accurate map of how quickly galaxies are approaching or receding away from us. We can still tell that things appear to be receding away from a central point, because we appear to be the central point. And also, the further away the galaxies are, the faster they appear to be moving away from us. Relying only on the radial velocity measurements, the whole universe seems to be receding away from specifically us! This fact could cause some existential alarm until one remembers the cosmological principle: that viewed on a sufficiently large scale, the properties of the universe should appear the same to any observer – it is more or less homogeneous. This is a strong claim, but is also one that has been born out by observations of the Cosmic Microwave Background (leftover radiation from the Big Bang).

If the universe looks similar to observers in different places, then the only possible conclusion from our observations of galaxies receding from us is that every galaxy is receding away from every other galaxy. This is the same as saying that the entire universe is expanding. Another consequence is that this expansion can’t be oriented around a single, central point. If we could construct the full 3-dimensional velocity vectors of every other galaxy, they would not point back to any particular location we could point to and say “That right there is the center of the universe!” Instead, everywhere is equally the center of the expansion. The Big Bang happened at all places, equally, and everything has been expanding away from everything else, in all directions, ever since. This is a direct consequence of the one dimension of the velocity vector which we can measure very well (the radial component) combined with the cosmological principle. And again, the fact that more distant galaxies appear to be moving more quickly away from us than nearby ones is exactly what you would expect to see in an expanding universe.

The rate of this expansion at early times (measured at the distant universe) compared to later times (measured at the more local universe) has recently come into tension, and seems to be pointing at brand new physics.

## If Aliens Exist, Here’s How We’ll Find Them

S uppose aliens existed, and imagine that some of them had been watching our planet for its entire four and a half billion years. What would they have seen? Over most of that vast timespan, Earth’s appearance altered slowly and gradually. Continents drifted ice cover waxed and waned successive species emerged, evolved, with many of them becoming extinct.

But in just a tiny sliver of Earth’s history—the last hundred centuries—the patterns of vegetation altered much faster than before. This signaled the start of agriculture—and later urbanization. The changes accelerated as the human population increased.

Then came even faster changes. Within just a century, the amount of carbon dioxide in the atmosphere began to rise dangerously fast. Radio emissions that couldn’t be explained by natural processes appeared and something else unprecedented happened: Rockets launched from the planet’s surface escaped the biosphere completely. Some spacecraft were propelled into orbits around the Earth others journeyed to the moon, Mars, Jupiter, and even Pluto.

If those hypothetical aliens continued to keep watch, what would they witness in the next century? Will a final spasm of activity be followed by silence due to climate change? Or will the planet’s ecology stabilize? Will there be massive terraforming? Will an armada of spacecraft launched from Earth spawn new oases of life elsewhere?

LASER POWER: A crucial impediment to space flight is the inefficiency of chemical fuel. One day a laser power station, located on Earth, might generate a beam to “push” a craft through space. NASA / Pat Rawlings (SAIC)

Let’s think specifically about the future of space exploration. Successful missions such as Viking, Cassini, New Horizons, Juno, and Rosetta were all done with last-century technology. We can realistically expect that during this century, the entire solar system—planets, moons, and asteroids—will be explored by flotillas of robotic craft.

Will there still be a role for humans in crewed spacecraft?

There’s no denying that NASA’s new Perseverance rover speeding across the Jezero crater on Mars may miss some startling discoveries that no human geologist could reasonably overlook. But machine learning is advancing fast, as is sensor technology. In contrast, the cost gap between crewed and autonomous missions remains huge.

We believe the future of crewed spaceflight lies with privately funded adventurers like SpaceX and Blue Origin, prepared to participate in a cut-price program far riskier than western nations could impose on publicly supported projects. These ventures—bringing a Silicon-Valley-type culture into a domain long-dominated by NASA and a few aerospace conglomerates—have innovated and improved rocketry far faster than NASA or the European Space Agency have done. The future role of the national agencies will be attenuated—becoming more akin to an airport rather than to an airline.

We are near the end of Darwinian evolution, but technological evolution of intelligent beings is just beginning.

We would argue that inspirationally led private companies should front all missions involving humans as cut-price high-risk ventures. There would still be many volunteers—a few perhaps even accepting one-way tickets—driven by the same motives as early explorers and mountaineers. The phrase “space tourism” should be avoided. It lulls people into believing such ventures are routine and low-risk. If that’s the perception, the inevitable accidents will be as traumatic as those of the space shuttle were. These exploits must be sold as dangerous, extreme sports, or intrepid exploration.

The most crucial impediment to space flight stems from the intrinsic inefficiency of chemical fuel, and the requirement to carry a weight of fuel far exceeding that of the payload. So long as we are dependent on chemical fuels, interplanetary travel will remain a challenge. Nuclear power could be transformative. Allowing much higher in-course speeds would drastically cut the transit times in the solar system, reducing not only astronauts’ boredom, but their exposure to damaging radiation. It’s more efficient if the fuel supply can be on the ground for instance, propelling spacecraft into orbit via a “space elevator”—and then using a “star-shot”-type laser beam generated on Earth to push on a “sail” attached to the spacecraft.

By 2100, thrill seekers in the mold of Felix Baumgartner (the Austrian skydiver who in 2012 broke the sound barrier in free fall from a high-altitude balloon) may have established bases on Mars, or maybe even on asteroids. Elon Musk has said he wants to die on Mars—“but not on impact.” It’s a realistic goal, and an alluring one to some.

But don’t expect mass emigration from Earth. It’s a dangerous delusion to think that space offers an escape from Earth’s problems. We’ve got to solve those here. Coping with climate change or the COVID-19 pandemic may seem daunting, but it’s a piece of cake compared to terraforming Mars. There’s no place in our solar system that offers an environment even as clement as the Antarctic or the top of Mount Everest. Simply put, there’s no Planet B for ordinary risk-averse people.

Still, we (and our progeny here on Earth) should cheer on the brave space adventurers. They have a pivotal role to play in spearheading the post-human future and determining what happens in the 22nd century and beyond.

P ioneer explorers will be ill-adapted to their new habitat, so they will have a compelling incentive to re-design themselves. They’ll harness the super-powerful genetic and cyborg technologies that will be developed in coming decades. This might be the first step toward divergence into a new species.

Organic creatures need a planetary surface environment on which life could emerge and evolve. But if post-humans make the transition to fully inorganic intelligence, they won’t need an atmosphere. They may even prefer zero-gravity, especially for constructing massive artifacts. It’s in deep space that non-biological brains may develop powers that humans can’t even imagine.

There are chemical and metabolic limits to the size and processing power of organic brains. Maybe we are close to these limits already. But no such limits apply to or constrain electronic computers (still less, perhaps, quantum computers). So, by any definition of “thinking,” the amount and intensity that can be achieved by organic human-type brains will be swamped by the cerebrations of AI.

We are perhaps near the end of Darwinian evolution, but technological evolution of intelligent beings is only just beginning. It may happen fastest away from Earth—we wouldn’t expect (and certainly wouldn’t wish for) such rapid changes in humanity here on the Earth, though our survival may depend on ensuring the AI on Earth remains “benevolent.”

SPACE TOURISM: Private space companies should avoid the phrase “space tourism,” write Martin Ress and Mario Livio. It lulls people into believing such ventures are routine and low-risk. They’re more like extreme sports. NASA / Jet Propulsion Laboratory

Few doubt machines will gradually surpass or enhance more and more of our distinctively human capabilities. Disagreements are only about the timescale on which this will happen. Inventor and futurist Ray Kurzweil says it will be just a matter of a few decades. More cautious scientists envisage centuries. Either way, the timescales for technological advances are an instant compared to the timescales of the Darwinian evolution that led to humanity’s emergence—and more relevantly, less than a millionth of the vast expanses of cosmic time ahead. The products of future technological evolution could surpass humans by as much as we have surpassed slime mold.

But, you may wonder, what about consciousness?

Philosophers and computer scientists debate whether consciousness is something that characterizes only the type of wet, organic brains possessed by humans, apes, and dogs. Would electronic intelligences, even if their intellects would seem superhuman, lack self-awareness? The ability to imagine things that do not exist? An inner life? Or is consciousness an emergent property that any sufficiently complex network will eventually possess? Some say it’s irrelevant and semantic, like asking whether submarines can swim.

Extraterrestrial civilization might consist of a swarm of microscopic probes that could have evaded notice.

We don’t think it is. If the machines are what computer scientists refer to as “zombies,” we would not accord their experiences the same value as ours, and the post-human future would seem rather bleak. On the other hand, if they are conscious, we should welcome the prospect of their future hegemony.

What will their guiding motivation be if they become fully autonomous entities? We have to admit we have absolutely no idea. Think of the variety of bizarre motives (ideological, financial, political, egotistical, and religious) that have driven human endeavors in the past. Here’s one simple example of how different they could be from our naive expectations: They could be contemplative. Even less obtrusively, they may realize it’s easier to think at low temperatures, therefore getting far away from any star, or even hibernating for billions of years until the cosmic microwave background cooled down far below its current 3 degrees Kelvin. At the other edge of the spectrum, they could be expansionist, which seems to be the expectation of most who’ve thought about the future trajectory of civilizations.

Even if life had originated only on Earth, it need not remain a marginal, trivial feature of the cosmos. Humans could jump-start a diaspora whereby ever-more complex intelligence spreads through the galaxy, transcending our limitations. The “sphere of influence” (or some would envisage a “frontier of conquest”) could encompass the entire galaxy, spreading via self-reproducing machines, transmitting DNA or instructions for 3-D printers. The leap to neighboring stars is just an early step in this process. Interstellar voyages—or even intergalactic voyages—would hold no terrors for near-immortals.

Moreover, even if the only propellants used were the currently known ones, this galactic colonization would take less time, measured from today, than the more than 500 million years elapsed since the Cambrian explosion. And even less than the 55 million years since the advent of primates, if it proceeds relativistically.

The expansionist scenarios would have the consequence that our descendants would become so conspicuous that any alien civilization would become aware of them.

T he crucial question remains: Are there other expansionists whose domain may impinge on ours?

We don’t know. The emergence of intelligence may require such a rare chain of events and happenstance contingencies—like winning a lottery—that it has not occurred anywhere else. That will disappoint SETI searchers and explain the so-called Fermi Paradox—the surprise expressed by physicist Enrico Fermi over the absence of any signs for the existence of other intelligent civilizations in the Milky Way. But suppose we are not alone. What evidence would we expect to find?

WE’RE LISTENING: The Allen Telescope Array, located at the Hat Creek Observatory in the Cascade Mountains, about 300 miles north of San Francisco, makes astronomical observations and stays attuned to signs of extraterrestrial life. Seth Shostak / SETI Institute

Suppose that there are indeed many other planets where life emerged, and that on some of them Darwinian evolution followed a similar track to the one on Earth. Even then, it’s highly unlikely that the key stages would be synchronized. If the emergence of intelligence and technology on a planet lags significantly behind what has happened on Earth (because, for example, the planet is younger, or because some bottlenecks in evolution have taken longer to negotiate) then that planet would reveal no evidence of ET. Earth itself would probably not have been detected as a life-bearing planet during the first 2 billion years of its existence.

But around a star older than the sun, life could have had a head start of a billion years or more. Note that the current age of the solar system is about half the age of our galaxy and also half of the sun’s predicted total lifetime. We expect that a significant fraction of the stars in our galaxy are older than the sun.

The history of human technological civilization is measured in mere millennia. It may be only a few more centuries before humans are overtaken or transcended by inorganic intelligence, which will then persist, continuing to evolve on a faster-than-Darwinian timescale for billions of years. Organic human-level intelligence may be, generically, just a brief interlude before the machines take over, so if alien intelligence had evolved similarly, we’d be most unlikely to catch it in the brief sliver of time when it was still embodied in that form. Were we to detect ET, it would be far more likely to be electronic where the dominant creatures aren’t flesh and blood—and perhaps aren’t even tied to a planetary surface.

Astronomical observations have now demystified many of the probability factors in the so-called Drake Equation—the probabilistic attempt traditionally used to estimate the number of advanced civilizations in the Milky Way. The number of potentially habitable planets has changed from being completely unknown only a couple of decades ago to being directly determined from the observations. At the same time, we must reinterpret one of the key factors in the Drake equation. The lifetime of an organic civilization may be millennia at most. But its electronic diaspora could continue for billions of years.

It’s in deep space that non-biological brains may develop powers that humans can’t even imagine.

If SETI succeeded, it would then be unlikely that the signal would be a decodable message. It would more likely reveal a byproduct (or maybe even a malfunction) of some super-complex machine beyond our comprehension.

The habit of referring to “alien civilizations” may in itself be too restrictive. A civilization connotes a society of individuals. In contrast, ET might be a single integrated intelligence. Even if messages were being transmitted, we may not recognize them as artificial because we may not know how to decode them, in the same way that a veteran radio engineer familiar only with amplitude-modulation (AM) transmission might have a hard time decoding modern wireless communications. Indeed, compression techniques aim to make the signal as close to noise as possible insofar as a signal is predictable, there’s scope for more compression.

SETI so far has focused on the radio part of the spectrum. But we should explore all wavebands, including the optical and X-ray band. We should also be alert for other evidence of non-natural phenomena or activity. What might then be a relatively generic signature? Energy consumption, one of the potential hallmarks of an advanced civilization, appears to be hard to conceal.

One of the most plausible long-term energy sources available to an advanced technology is starlight. Powerful alien civilizations might build a mega-structure known as a “Dyson Sphere” to harvest stellar energy from one star, many stars, or even from an entire galaxy.

The other potential long-term energy source is controlled fusion of hydrogen into heavier nuclei. In both cases, waste heat and a detectable mid-infrared signature would be an inevitable outcome. Or, one might seek evidence for massive artifacts such as the Dyson Sphere itself. Intriguingly, it’s worth looking for artifacts within our own solar system: Maybe we can rule out visits by human-scale aliens, but if an extraterrestrial civilization had mastered nanotechnology and transferred its intelligence to machines, the “invasion” might consist of a swarm of microscopic probes that could have evaded notice. Still, it would be easier to send a radio or laser signal than to traverse the mind-boggling distances of interstellar space.

F inally, let’s fast forward not for just a few millennia, but for an astronomical timescale, millions of times longer. As interstellar gas will be consumed, the ecology of stellar births and deaths in our galaxy will proceed more gradually, until jolted by the environmental shock of a collision with the Andromeda galaxy, about 4.5 billion years hence. The debris of our galaxy, Andromeda, and their smaller companions (known as the Local Group) will aggregate into one amorphous (or perhaps elliptical) galaxy. Due to the accelerating cosmic expansion, distant galaxies will move farther away, receding faster and faster until they disappear—rather like objects falling into a black hole—encountering a horizon beyond which they are lost from view and causal contact. But the remnants of our Local Group could continue for a far longer time. Long enough perhaps for what has been dubbed a “Kardashev Type III” phenomenon, in which a civilization is using the energy from one or more galaxies, and perhaps even that released from supermassive black holes, to emerge as the culmination of the long-term trend for living systems to gain complexity and negative entropy (a higher degree of order).

The only limitations set by fundamental physics would be the number of accessible protons (since those can in principle be transmuted into any elements), and the total amount of accessible energy (E=mc 2 , where m is mass and c is the speed of light) again transformable from one form to another.

Essentially all the atoms that were once in stars and gas could be transformed into structures as intricate as a living organism or silicon chips but on a cosmic scale. A few science-fiction authors envisage stellar-scale engineering to create black holes and wormholes—concepts far beyond any technological capability that we can imagine, but not in violation of basic physical laws.

#### Point and Shoot

The history of human exploration has always been tied to the search for a destination. Space travel is no different: The Apollo program, which ferried humans to the moon, was not only a scientific and technical achievement, but also an. READ MORE

If we want to go to further extremes, the total mass-energy content in the Local Group isn’t the limit of the available resources. It would still be consistent with physical laws for an incredibly advanced civilization to lasso the galaxies that are receding because of the cosmic expansion of space before they accelerate and disappear over the horizon. Such a hyper-intelligent species could pull them in to construct a segment resembling Einstein’s original idea of a static universe in equilibrium, with a mean density such that the cosmic repulsion caused by dark energy is precisely balanced by gravity.

Everything we’ve said is consistent with the laws of physics and the cosmological model as we understand them. Our speculations assume that the repulsive force causing cosmic acceleration persists (and is described by dark energy or Einstein’s cosmological constant). But we should be open-minded about the possibility that there is much we don’t understand.

Human brains have changed relatively little since our ancestors roamed the African savannah and coped with the challenges that life then presented. It is surely remarkable that these brains have allowed us to make sense of the quantum subatomic world and the cosmos at large—far removed from the common sense, everyday world in which we have evolved.

Scientific frontiers are now advancing fast. But we may at some point hit the buffers. There may be phenomena, some of which may be crucial to our long-term destiny, that we are not aware of any more than a gorilla comprehends the nature of stars and galaxies. Physical reality could encompass complexities that neither our intellect nor our senses can grasp. Electronic brains may have a rather different perception of reality. Consequently, we cannot predict or perhaps even understand the motives of such brains. We cannot assess whether the Fermi paradox signifies their absence or simply their preference.

Conjectures about advanced or intelligent life are shakier than those about simple life. Yet there are three features that may characterize the entities that SETI searches could reveal.

• Intelligent life is likely not to be organic or biological.

• It will not remain on the surface of the planet where its biological precursor emerged and evolved.

• We will not be able to fathom the intentions of such life forms.

Two familiar maxims should pertain to all SETI searches. On one hand, “absence of evidence isn’t evidence of absence,” but on the other, “extraordinary claims require extraordinary proof.

Martin Rees is Astronomer Royal for the United Kingdom and author of On the Future.

Mario Livio is an astrophysicist and author of Galileo and the Science Deniers.

## How fast is the universe expanding? We know nought.

Astronomers have been trying to pin down the rate the universe is expanding instead of relenting under a century of experimental scrutiny, it is playing us at “whack a mole.” (Photo: NASA via AP)

Scientists love to be wrong. When we are wrong, something new is to be learned about nature. What could be more exciting to be wrong about than the universe itself? The science of the universe is called cosmology — and something seems quite broken with ours.

It started 100 years ago when Edwin Hubble discovered the universe is expanding. In the 1920s, he determined that, the further away a galaxy, the faster it is racing from us. The rate of cosmic expansion is measured as Hubble’s constant, H0 (pronounced H-nought). Astronomers have been trying to pin down this rate instead of relenting under a century of experimental scrutiny, it is playing us at “whack a mole.”

From the 1950s to 2000s, H0 bounced between 50 and 100 km/s/Mpc. What is a “km/s/Mpc?” A “km/s” means kilometers per second, a measure of speed. A “Mpc” means Megaparsec, a measure of distance (roughly 3 million lightyears, where one lightyear is the distance light travels in a year, about 6 billion miles). If H0=100 km/s/Mpc, it means that the space between two galaxies separated by a Mpc is expanding such that the galaxies are carried away from each other at 100 km/s (that speed could get you to the moon in an hour).

Chris Churchill (Photo: NMSU Astronomy Department)

In the 2000s, astronomers leveraged new insights on the nature of exploding stars called supernovae to definitively pin down H0. But, the universe yelled back, “wrong!” The universe is not passively expanding its expansion is accelerating! This forced us to introduce “dark energy” into our equations of cosmology — a lot of dark energy. Roughly 70% of the universe must be dark energy. It acts like a smooth, unchanging, ubiquitous repulsive force, everywhere in the universe, but nowhere. No one understands what it actually is.

Over the last decade, two teams continued slugging it out with H0.

The team led by Adam Reiss, called “Supernovae, H0, and the Equation of State of Dark Energy” (SHOES), uses the supernova method. This involves very direct measurements of the distances to galaxies and the speeds at which they are receding — basically Hubble’s original 1920 experiment supercharged with modern technology, space telescopes and an additional century of astrophysics under our belts. SHOES measured H0=73.5 km/s/Mpc with a 2% error!

Whereas SHOES directly revealed the rate the universe is expanding today, the second method is more complex. A team led by Nazzareno Mandolesi and Jean-Loup Puget used the Planck space telescope to measure patterns in the light echoes from the beginning of the universe and compared them to the patterns of how galaxies are distributed in the universe today. That is a lot of info, so let’s unpack it. The ancient light echoes tell us how matter behaved in the early universe. In a delicate balance, like a mass on a spring, this matter “oscillated” around denser pockets. The sizes of these tiny oscillations became “frozen” in space and then grew to be 150 Mpc across as the universe expanded over the next 13 billion years. The Planck team measured the sizes in the early universe and compared them to the sizes in the present day universe. The latter were measured using the decade-long experiment called the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), which employed the Sloan Telescope at New Mexico State University’s Apache Point Observatory near Cloudcroft. The Planck team measured H0=67.6 km/s/Mpc with a 0.4% error!!

What is the expansion rate of the universe? Scientific studies give different answers. (Photo: Courtesy)

Though H0=73.5 km/s/Mpc (SHOES) and H0=67.6 km/s/Mpc (Planck) seems like pretty good agreement, the differences are highly significant there is a 1/7,000 chance these precision experiments can be resolved. This is the crisis in cosmology.

Either one of these trusted experiments is wrong or our understanding of the universe is wrong. A month ago SHOES incorporated new precision calibration data from a space telescope called Gaia to refine the value to H0=73.2 km/s/Mpc. No change there. Perhaps the Plank results are suspect because we are wrong about dark energy? The problem is, the eBOSS experiment has pinned down its mathematical behavior over the last 8 billion years.

So here we are. Many astrophysicists are tantalized by the possibility that we have our cosmology wrong. It has happened twice before. After 2,000 years of Aristotle’s Earth-centered universe, Copernicus moved the sun to the center. In the 1960s, Fred Hoyle proposed the “Steady State” universe, in which there was no beginning — the universe just expanded at the same rate forever and new matter was “spontaneously” generated between the galaxies. Now, we favor the Big Bang universe. It is a highly successful mathematical cosmology that has made successful predictions and survived many experimental tests. But, it has required significant modifications to keep it working (dark matter and dark energy) … and now it is being challenged like never before by that pesky H0 — right where it all started. Exciting indeed!

## Elementary Astronomy (107)

We know what it's like to be inside a spiral galaxy, because we live in one, the Milky Way galaxy. The Sun lies near the small Orion Arm or "Spur", and between the Satittarius and Perseus Arms. Spiral arms of our galaxy are named for the constellations toward which we look to see them. The Milky Way has a small central bar so it is actually a SBbc barred spiral.

The Milky Way mapped using infrared images from NASA's Spitzer Space Telescope.
Credit: NASA/JPL-Caltech

NGC 4622 spiral galaxy

Sombrero (M104) spiral galaxy

ESO 510-G13 warped spiral galaxy

A spiral galaxy has a flattened disk of stars but also a spherical glowing halo. The stars in the disk are young, typically recently formed, and are different from those in the halo. Halo stars are small, as much as 13 billion years old, made early in the galaxy's history, early in the history of the Universe! They are so old that they are metal-poor, having been made before other larger stars exploded and sent metals out into the galaxy.

You can see that the halo in these images looks yellow or even slightly red compared to the bluer disk. Blue stars are more massive, and age quickly. Only the less massive red stars remain in the aging halo population. The presence of obscuring dust, dark molecular clouds, and bright emission nebulae is characteristic of regions of the galaxy were stars are being formed now.

Our Milky Way, the Andromeda galaxy (M31), and Triangulum (M33) are the only spiral galaxies in the Local Group of Galaxies .

Panoramic montage of the Milky Way Galaxy in our night sky
note Andromeda Galaxy tiny on lower left
Credit: Axel Mellinger

Andromeda Galaxy (M31) is a spiral a little smaller than our Milky Way

Local Group of Galaxies

##### ELLIPTICAL GALAXIES

NGC 1316 is an enormous elliptical galaxy

Close-up of center of NGC 1316 elliptical galaxy

Elliptical galaxies are classified, roundest to most oblong, as E0, E2, E3, E4, E5, E6, E7. Elliptical galaxies really are ellipsoids, somewhat football-shaped, and quite unlike the flat disks of spiral galaxies that, seen at an angle, also may appear elliptical.

M87 in the Virgo cluster is close enough to us that we can see details that provide clues
about the origin of its unusual 5000 light year long jet of electrons.
Credit: Tod R. Lauer, Sandra Faber, NASA
##### BARRED SPIRAL GALAXIES

Barred spiral galaxy NGC 1672
Credit: NASA, ESA,and the Hubble Heritage Team
##### EDWIN HUBBLE'S CLASSIFICATIONS OF ELLIPTICAL, SPIRAL, AND BARRED SPIRAL GALAXIES

Elliptical galaxies from E0 to E7, spiral galaxies from Sa to Sc, barred spiral galaxies from SBa to SBc

##### IRREGULAR GALAXIES

Irregular galaxies do not fit into the Hubble "tuning fork" and have no specific shape. Sometimes it is difficult to decide whether there is underlying structure. The Large and Small Magellanic Clouds, for example, are irregular in appearance but also have a bar that suggests they may be unusual small barred spirals.

Large Magellanic Cloud on the left and Small Magellanic Cloud on the right

The small irregular galaxy NGC 1427A, seen in this image from the Hubble Space Telescope, is plunging into the Fornax cluster of galaxies at 600 km/s. Within the next billion years it will be completly disrupted by the impending collision.

##### SOMETIMES GALAXIES COLLIDE

Whirlpool galaxy (M51) near the tail of the Big Bear

Arp 87 colliding galaxies

The Antennae galaxies were separate galaxies 1.2 billion years ago. Simulations of the collision predict that within 400 million years their nuclei will form a single core, and that the system will evolve into an elliptical galaxy.

Antennae galaxies colliding

It seems frightening that our own Milky Way and the Andromeda galaxy may collide in 3 billion years, until we notice that, even as we speak, the Milky Way is already colliding with a small elliptical galaxy, Sagittarius Dwarf galaxy.

Sagittarius Dwarf galaxy
##### QUASARS

Quasars were named "quasi-stellar" because they appeared as mysterious radio sources and bright points of light, star-like, but with unusual spectra. These points of light were extremely bright nuclei of galaxies fueled by supermassive black holes a billion times as massive as normal black holes! With better telescopes and cameras we may now see the fainter galaxies around these bright cores. Quasars are also called active galaxies because the black hole is still thrashing infalling matter into vortices that emit enormous amounts of energy and eject gas out thousands of light years into the space beyond the visible galaxy.

Quasars are in the furthest reaches of the visible universe, created when the universe was compact and dense, the first throws of modern galaxies. They appear much like M87, only millions of times brighter at the central nucleus. Below we see a distant quasar, 10 billion light years (3 billion parsecs) away, through a cluster of galaxies "only" 7 billion light years away. The cluster's gravity acts as a lens, focusing the light from the bright yellow quasar and its super bright core into 4 bright blue-white star-like spots.

The four bright blue-white points are actually images of a single distant quasar made visible by the gravity of a cluster of galaxies that bends the light on its way to us: a gravitational lens
Credit: K. Sharon and E. Ofek, ESA, NASA

##### THE MOST DISTANT GALAXIES

Galaxies seen by Hubble Deep Field (HDF) above
Credits: R. Williams, the HDF Team, NASA

These images were assembled from hundreds of separated exposures with the Hubble Telescope pointing in one direction, creating a view to the visible horizon of the universe. There is an assortment of thousands of galaxies in at various stages of evolution in these images.

Distant galaxies are also being studied from the ground, in methodical surveys covering wider spans of the sky. The Sloan Digital Sky Survey has been accumulating images and spectra continuously since 2000, and provides a picture of the distribution and types of galaxies from those nearby out to a redshift of z=6 for the distant quasars. You can participate in exploring and classifying the galaxies they are finding by joining the Galaxy Zoo project.

#### WHERE THE GALAXIES STOP

##### THE FIRST STARS APPEARED BEFORE THERE WERE GALAXIES

About 100 million years after the Big Bang
When pressure and temperature were right for something to happen ..
It will

##### AND BEFORE THE STARS THE DARK AGE

Which wasn't really dark at all.

Meanwhile, little by little, slightly denser regions in the gas of newly formed hydrogen and helium atoms gradually drew together until they formed into super-giant, super-hot, blue stars. The Dark Age came to an end with the formation of these first stars.

The Dark Age, before stars, wasn't really dark and might have looked like this.
At nearly 1000 K the universe at that time glowed with red and near infrared and light, the color and same temperature of an electric hotplate seen in a dark room.

Cosmic Background Radiation was first thought to be noise and blamed it on two nesting pigeons in this Bell Horn Antenna

Before the Dark Age, the Universe was so hot that protons and neutrons could not hold onto electrons, so there were no atoms. Not until, 300,000 years after the Big Bang, the Universe and expanded and cooled to less than 3,000 K. These less energetic photons and weaker collisions among the particles allowed protons and neutrons to hold onto their electrons,. The first atoms of hydrogen and helium formed, and stayed intact.

Previously photons had been absorbed by the free electrons almost as soon as they had been emitted, like photons are caught inside stars taking thousands of years to get out. Now the Universe was cool enough to be transparent to photons, and they were suddenly set free to travel vast distances at the speed of light.

As the Universe expanded, the Cosmic Background light was carried with it. To us today, the Background "Radiation" appears red-shifted by z=1000 times, making the wavelengths appear 1000 times longer. Though emitted with wavelengths about 1,000 nm, in the near infrared at a temperature about 3000 K, the redshift makes the wavelengths of the photons much longer, "lazy", and less energetic. They now seem to be coming from a cold 3 K source (2.725 degrees above absolute zero), and they have shifted in wavelength so far, they are "microwaves" with wavelengths of about 1,000,000 nm (1 mm).

The Cosmic Background Radiation was accidentally discovered at the Bell Horn Antenna, where it was thought it might be noise caused by two nesting pigeons. If you want to watch something on your old analog television sets, you can always tune in the uh, educational Cosmic Background Radiation show, which appears as static on those channels being abandoned by the broadcast stations.

The Cosmic Microwave Background radiation is virtually the same in all directions, with only miniscule variations in apparent temperature. In the 1970's it was also found to be slightly "hotter" in one direction in space, a difference of about 0.1% of the apparent temperature. This was because it was slightly more red-shifted for viewing in one direction and blue-shifted for viewing in the opposite direction. This is the exact pattern that would be expected if our local group of galaxies were moving through the background radiation at a speed about 600 km/s! In a sense, we can tell which direction we are moving compared to the gas of hydrogen that made the first stars.

Originally emitted as near infrared light at 3000 K, light scattered off of the electrons and protons of the early universe appears to us today with
a redshift of z = 1000.
Credit: NASA/WMAP Science Team

#### BEFORE THE COSMIC BACKGROUND RADIATION

In the first 300,000 years of the Universe, temperatures cooled from 10 27 K (that's 1,000,000,000,000,000,000,000,000,000 K, a trillion quadrillion Kelvin) down to 10,000 K.

With such extreme heat, photons in the form of ultraviolet light, x-rays and gamma rays made the universe extremely bright. It would have been like being inside of the star Sirius.

History of the Universe from the Big Bang

• to a super-bright super-hot expansion period
• to Stars
• to Galaxies with Stars with Planets
• to Life

#### The First 3 Minutes of the Universe

And as we've seen before, when temperatures and pressures are right for something to happen . it will.

The first 10 -43 second is obscured by our lack of knowledge of how gravity works. By 10 -32 seconds the universe had inflated enormously and the first particles appeared.
By 10 -5 seconds quarks linked up in trios to make protons.
By 10 -4 seconds the universe was left with 5x more protons than neutrons (protons being hydrogen nuclei).
After one second the universe had expanded enough that it became transparent to neutrinos, and they went on their own way forever.
After 3 minutes and 42 seconds, the universe had cooled enough for helium nuclei to be stable, and it had 25% of its ordinary matter in that form.
After 1 year, its temperature was about that in the center of a star like the Sun, cooling gradually.

After 300,000 years, light could get through it, to become the cosmic background we see today. The matter that remained began to coalesce under its own gravity, and the first stars appeared after a million years . and eventually us.

The Universe began with energy that became photons and particles
E=mc 2 also means E/c 2 = m

( This image is also useful in checking your eyes for astigmatism, and to see if your eye glasses are correct.)

Hmm, there seem to be a few things missing here.

• If galaxies appear 13 billion ly away, but their light left the redshifting galaxies 13 billion years ago . how big is the Universe now?
• When something appears to recede with a redshift of z = 1000, does time change ?
• Dark energy seems to be making the Universe inflate faster and faster, while dark matter seems to be holding it together, but we don't know what either one is.
• Did time exist before the Big Bang?
• If gravity around massive objects alters space and time, what happens around a black hole?

These require Special and General Relativity, quantum theory, and perhaps physics we do not yet know to answer !

## Search for 'dark energy' could illuminate origin, evolution, fate of universe

The Hobby-Eberly Telescope. Credit: Marty Harris, McDonald Observatory, UT Austin

The universe we see is only the very tip of the vast cosmic iceberg.

The hundreds of billions of galaxies it contains, each of them home to billions of stars, planets and moons as well as massive star-and-planet-forming clouds of gas and dust, and all of the visible light and other energy we can detect in the form of electromagnetic radiation, such as radio waves, gamma rays and X-rays—in short, everything we've ever seen with our telescopes—only amounts to about 5% of all the mass and energy in the universe.

Along with this so-called normal matter there is also dark matter, which can't be seen, but can be observed by its gravitational effect on normal, visible matter, and makes up another 27% of the universe. Add them together, and they only total 32% of the mass of the universe—so where's the other 68%?

So what exactly is dark energy? Put simply, it's a mysterious force that's pushing the universe outward and causing it to expand faster as it ages, engaged in a cosmic tug-of-war with dark matter, which is trying to pull the universe together. Beyond that, we don't yet understand what dark energy is, but Penn State astronomers are at the core of a group that's aiming to find out through a unique and ambitious project 16 years in the making: HETDEX, the Hobby-Eberly Telescope Dark Energy Experiment.

"HETDEX has the potential to change the game," said Associate Professor of Astronomy and Astrophysics Donghui Jeong.

Dark energy and the expanding universe

Today there is consensus among astronomers that the universe we inhabit is expanding, and that its expansion is accelerating, but the idea of an expanding universe is less than a century old, and the notion of dark energy (or anything else) accelerating that expansion has only been around for a little more than 20 years.

In 1917 when Albert Einstein applied his general theory of relativity to describe the universe as a whole, laying the foundations for the big bang theory, he and other leading scientists at that time conceived of the cosmos as static and nonexpanding. But in order to keep that universe from collapsing under the attractive force of gravity, he needed to introduce a repulsive force to counteract it: the cosmological constant.

It wasn't until 1929 when Edwin Hubble discovered that the universe is in fact expanding, and that galaxies farther from Earth are moving away faster than those that are closer, that the model of a static universe was finally abandoned. Even Einstein was quick to modify his theories, by the early 1930s publishing two new and distinct models of the expanding universe, both of them without the cosmological constant.

But although astronomers had finally come to understand that the universe was expanding, and had more or less abandoned the concept of the cosmological constant, they also presumed that the universe was dominated by matter and that gravity would eventually cause its expansion to slow the universe would either continue to expand forever, but ever-increasingly slowly, or it would at some point cease its expansion and then collapse, ending in a "big crunch."

"That's the way we thought the universe worked, up until 1998," said Professor of Astronomy and Astrophysics Robin Ciardullo, a founding member of HETDEX.

That year, two independent teams—one led by Saul Perlmutter at Lawrence Berkeley National Laboratory, and the other led by Brian Schmidt of the Australian National University and Adam Riess of the Space Telescope Science Institute—would nearly simultaneously publish astounding results showing that the expansion of the universe was in fact accelerating, driven by some mysterious antigravity force. Later that year, cosmologist Michael Turner of the University of Chicago and Fermilab coined the term "dark energy" to describe this mysterious force.

The discovery would be named Science magazine's "Breakthrough of the Year" for 1998, and in 2011 Perlmutter, Schmidt and Reiss would be awarded the Nobel Prize in physics.

This pie chart shows rounded values for the three known components of the universe: normal matter, dark matter, and dark energy. Credit: NASA's Goddard Space Flight Center

More than 20 years after the discovery of dark energy, astronomers still don't know what, exactly, it is.

"Whenever astronomers say 'dark," that means we don't have any clue about it," Jeong said with a wry grin. "Dark energy is just another way of saying that we don't know what's causing this accelerating expansion."

There are, however, a number of theories that attempt to explain dark energy, and a few major contenders.

Perhaps the most favored explanation is the previously abandoned cosmological constant, which modern-day physicists describe as vacuum energy. "The vacuum in physics is not a state of nothing," Jeong explained. "It is a place where particles and antiparticles are continuously created and destroyed." The energy produced in this perpetual cycle could exert an outward-pushing force on space itself, causing its expansion, initiated in the big bang, to accelerate.

Unfortunately, the theoretical calculations of vacuum energy don't match the observations—by a factor of as much as 10 120 , or a one followed by 120 zeroes. "That's very, very unusual," Jeong said, "but that's where we'll be if dark energy turns out to be constant." Clearly this discrepancy is a major issue, and it could necessitate a reworking of current theory, but the cosmological constant in the form of vacuum energy is nonetheless the leading candidate so far.

As a result of its design, HETDEX is collecting a massive amount of data, extending well beyond its intended targets and providing additional insights into things like dark matter and black holes, the formation and evolution of stars and galaxies, and the physics of high-energy cosmic particles such as neutrinos.

Another possible explanation is a new, yet-undiscovered particle or field that would permeate all of space but so far, there's no evidence to support this.

A third possibility is that Einstein's theory of gravity is incorrect. "If you start from the wrong equation," Jeong said, "then you get the wrong answer." There are alternatives to general relativity, but each has its own issues and none has yet displaced it as the reigning theory. For now, it's still the best description of gravity we've got.

Ultimately, what's needed is more and better observational data—precisely what HETDEX was designed to collect like no other survey has done before.

A map of stars and sound

"HETDEX is very ambitious," Ciardullo said. "It's going to observe a million galaxies to map out the structure of the universe going over two-thirds of the way back to the beginning of time. We're the only ones going out that far to see the dark energy component of the universe and how it's evolving."

Ciardullo, an observational astronomer who studies everything from nearby stars to faraway galaxies and dark matter, is HETDEX's observations manager. He's quick to note, though, that he's got help in that role (from Jeong and others) and that he and everyone else on the project wears more than one hat. "This is a very big project," he said. "It's over \$40 million. But if you count heads, it's not very many people. And so we all do more than one thing."

Jeong, a theoretical astrophysicist and cosmologist who also studies gravitational waves, was instrumental in laying the groundwork for the study and is heavily involved in the project's data analysis—and he's also helping Ciardullo determine where to point the 10-meter Hobby-Eberly Telescope, the world's third largest. "It's kind of interesting," he noted with a chuckle, "a theorist telling observers where to look."

This diagram shows the changes in the rate of expansion since the universe's birth. The shallower the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious force — dark energy — that is pulling galaxies apart. I. Credit: NASA/STScI/Ann Feild

While other studies measure the universe's expansion using distant supernovae or a phenomenon known as gravitational lensing, where light is bent by the gravity of massive objects such as galaxies and black holes, HETDEX is focused on sound waves from the big bang, called baryonic acoustic oscillations. Although we can't actually hear sounds in the vacuum of space, astronomers can see the effect of these primordial sound waves in the distribution of matter throughout the universe.

During the first 400,000-or-so years following the big bang, the universe existed as dense, hot plasma—a particle soup of matter and energy. Tiny disturbances called quantum fluctuations in that plasma set off sound waves, like ripples from a pebble tossed into a pond, which helped matter begin to clump together and form the universe's initial structure. The result of this clumping is evident in the cosmic microwave background (also called the "afterglow" of the big bang), which is the first light, and the farthest back, that we can see in the universe. And it's also imprinted in the distribution of galaxies throughout the universe's history—like the ripples on our pond, frozen into space.

"The physics of sound waves is pretty well known," Ciardullo said. "You see how far these things have gone, you know how fast the sound waves have traveled, so you know the distance. You have a standard ruler on the universe, throughout cosmic history."

As the universe has expanded so has the ruler, and those variances in the ruler will show how the universe's rate of expansion, driven by dark energy, has changed over time.

"Basically," Jeong said, "we make a three-dimensional map of galaxies and then measure it."

To make their million-galaxy map, the HETDEX team needed a powerful new instrument.

A set of more than 150 spectrographs called VIRUS (Visible Integral-Field Replicable Unit Spectrographs), mounted on the Hobby-Eberly Telescope, gathers the light from those galaxies into an array of some 35,000 optical fibers and then splits it into its component wavelengths in an ordered continuum known as a spectrum.

Galaxies' spectra reveal, among other things, the speed at which they are moving away from us—a measurement known as "redshift." Due to the Doppler effect, the wavelength of an object moving away from its observer is stretched (think of a siren that gets lower in pitch as it speeds away), and an object moving toward its observer has its wavelength compressed, like that same siren increasing in pitch as it gets nearer. In the case of receding galaxies, their light is stretched and thus shifted toward the red end of the spectrum.

Measuring this redshift allows the HETDEX team to calculate the distance to those galaxies and produce a precise three-dimensional map of their positions.

Among the galaxies HETDEX is observing are what are known as Lyman-alpha galaxies—young star-forming galaxies that emit strong spectral lines at specific ultraviolet wavelengths.

"We're using Lyman-alpha-emitting galaxies as a 'tracer particle,'" explained Research Professor of Astronomy and Astrophysics Caryl Gronwall, who is also a founding member of HETDEX. "They're easy to find because they have a very strong emission line, which is easy to find spectroscopically with the VIRUS instrument. So we have this method that efficiently picks out galaxies at a fairly high redshift, and then we can measure where they are, measure their properties."

Gronwall, who along with Ciardullo has been studying Lyman-alpha galaxies for nearly 20 years, leads HETDEX's efforts in this area, while Associate Professor of Astronomy and Astrophysics Derek Fox lends his expertise to calibrating the VIRUS instrument, using incidental observations of stars with well-known properties to fine-tune its spectra.

"Every shot we take with HETDEX, we observe some stars on the fibers," Fox explained. "That's an opportunity, because the stars are telling you how sensitive your experiment is. If you know the brightness of the stars and you see the data that you collect on them, it offers an opportunity to keep your calibration on point."

In this representation of the evolution of the universe, the far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth. The afterglow light (known as the cosmic microwave background) was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light, which also forms a backlight for later developments of the universe. Credit: NASA/WMAP Science Team

One of HETDEX's biggest strengths is that it was designed as a blind survey—observing broad swaths of sky instead of specific, predetermined objects. "Nobody has tried doing a survey like this before," Ciardullo said. "It's always "Find your objects, then do the spectroscopy." We're the first ones to try to do a whole lot of spectroscopy and then figure out what we saw."

As a result of this design, HETDEX is collecting a massive amount of data, extending well beyond its intended targets and providing additional insights into things like dark matter and black holes, the formation and evolution of stars and galaxies, and the physics of high-energy cosmic particles such as neutrinos.

"That's very different and very interesting," Jeong said. "We have huge discovery space."

Ciardullo added, "One thing you can infer—if you first have to see an object before pointing your spectroscope there, well that's fine, but it requires that the object be able to be seen. HETDEX can observe spectra of things that you can't see."

This means that in addition to the known data it's collecting, HETDEX is opening a window to unexpected findings, discoveries yet unforeseen. "We will be a pathfinder for more experiments," Ciardullo said, and that sentiment is echoed by others on the team, including Fox.

"We're definitely going to be blazing trails out there," he said. "There's big, big potential for really exciting discoveries."

Back to roots, and beyond

The futuristic science of HETDEX is, in a strange twist, very much in line with the ideas that drove the development of the Hobby-Eberly Telescope (HET) nearly 40 years ago.

"HET was initially conceived as the Penn State Spectroscopic Survey Telescope," explained Professor Emeritus of Astronomy and Astrophysics Larry Ramsey, who invented the telescope in 1983 with then Penn State colleague Dan Weedman, and later served as chairman of the HET's board of directors. "The original mission was to conduct spectroscopic surveys, and in the almost 20 years between when we first dedicated the telescope and when we started HETDEX, the telescope was not really doing surveys. So in a very real sense HETDEX is taking the HET back to its roots, and it has grown into a really interesting project."

"The scale of this survey is very futuristic, even now," Jeong said. Recalling a recent cosmology conference, he related a discussion about the future of galactic surveys. "I sat there and listened, and it was basically what we're doing," he said. "HETDEX is a future survey that exists now."

In addition to what HETDEX discovers about dark energy, the data it's collecting will also provide fodder for future studies far beyond the scope of its own mission. And chances are, HETDEX will continue doing "spacebreaking" science on the distant, high-redshift universe for quite a few years to come.

"Even currently planned future surveys don't go beyond HETDEX," Jeong said. "I think we will still be at the forefront, even 10 years from now."

## Redshifts, Blueshifts

Stars emit light. Using a prism or a diffraction grating, we can spread this light out into a spectrum. If we look at the spectrum of the Sun or any other star, we see not only the rainbow of colors from red, orange and yellow through to violet, but also a distinctive pattern of dark lines. At certain wavelengths, light will be ABSORBED by chemical elements like hydrogen, helium, calcium, and iron. These wavelengths are based on atomic physics and can be measured extremely accurately in a laboratory on Earth.

If a star is moving towards us, the whole pattern of the spectrum gets shifted to shorter wavelengths, i.e. towards the blue end of the spectrum. This is a BLUESHIFT, and we can measure it very accurately by comparing the apparent wavelengths of the spectral lines with the known laboratory wavelengths. If the star is receding, the pattern moves to longer, redder wavelengths, and this is a REDSHIFT. "Blueshifts come, and redshifts go, and that's pretty much everything you need to know."

v = c x (wavelength shift) / (wavelength)

where v is the relative velocity (speed) of the star, c is the velocity of light, and the known wavelength of the line as measured in the laboratory. We can use this equation until the relative speed becomes a significant fraction of the speed of light -- and then it's back to Einstein's Theory of Relativity, for a more detailed formula.

Extremely careful measurements of nearby stars can determine their relative speeds with a precision as small as 10 meters per second. Analyzing a large number of nearby stars shows that our Sun and its neighbors orbit the distant center of our Milky Way galaxy with a velocity of 250 km/sec. We're about 26,000 light years from the center of the Galaxy, and so it'll take 220 million years for us to complete an orbit. We can also tell that the stars nearer the center of the Galaxy rotate faster, so we're being 'passed' by those stars nearer the center, and 'overtaking' those stars that orbit further from the center of the Galaxy this is the DIFFERENTIAL ROTATION of the Galaxy.

## Earlier Evidence

Even before the discovery of quasars, there had been hints that something very strange was going on in the centers of at least some galaxies. Back in 1918, American astronomer Heber Curtis used the large Lick Observatory telescope to photograph the galaxy Messier 87 in the constellation Virgo. On that photograph, he saw what we now call a jet coming from the center, or nucleus, of the galaxy (Figure 6). This jet literally and figuratively pointed to some strange activity going on in that galaxy nucleus. But he had no idea what it was. No one else knew what to do with this space oddity either.

The random factoid that such a central jet existed lay around for a quarter century, until Carl Seyfert, a young astronomer at Mount Wilson Observatory, also in California, found half a dozen galaxies with extremely bright nuclei that were almost stellar, rather than fuzzy in appearance like most galaxy nuclei. Using spectroscopy, he found that these nuclei contain gas moving at up to two percent the speed of light. That may not sound like much, but it is 6 million miles per hour, and more than 10 times faster than the typical motions of stars in galaxies.

Figure 6. The M87 Jet: At top is an HST visible light image of M87 showing the huge jet of material streaming away from the bright, point-like nucleus located at upper left. The three panels at bottom show the jet at three different wavelengths. From left-to-right: image from the Chandra X-ray satellite, VLA radio image and another HST visible light image. In each image the nucleus is at lower left.

After decades of study, astronomers identified many other strange objects beyond our Milky Way Galaxy they populate a whole “zoo” of what are now called active galaxies or active galactic nuclei (AGN). Astronomers first called them by many different names, depending on what sorts of observations discovered each category, but now we know that we are always looking at the same basic mechanism. What all these galaxies have in common is some activity in their nuclei that produces an enormous amount of energy in a very small volume of space. In the next section, we describe a model that explains all these galaxies with strong central activity—both the AGNs and the QSOs.

### Key Concepts and Summary

The first quasars discovered looked like stars but had strong radio emission. Their visible-light spectra at first seemed confusing, but then astronomers realized that they had much larger redshifts than stars. The quasar spectra obtained so far show redshifts ranging from 15% to more than 96% the speed of light. Observations with the Hubble Space Telescope show that quasars lie at the centers of galaxies and that both spirals and ellipticals can harbor quasars. The redshifts of the underlying galaxies match the redshifts of the quasars embedded in their centers, thereby proving that quasars obey the Hubble law and are at the great distances implied by their redshifts. To be noticeable at such great distances, quasars must have 10 to 100 times the luminosity of the brighter normal galaxies. Their variations show that this tremendous energy output is generated in a small volume—in some cases, in a region not much larger than our own solar system. A number of galaxies closer to us also show strong activity at their centers—activity now known to be caused by the same mechanism as the quasars.

### Glossary

an object of very high redshift that looks like a star but is extragalactic and highly luminous also called a quasi-stellar object, or QSO

active galactic nuclei (AGN):

galaxies that are almost as luminous as quasars and share many of their properties, although to a less spectacular degree abnormal amounts of energy are produced in their centers