Astronomy

How do we know the universe's expansion is speeding up?

How do we know the universe's expansion is speeding up?


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Please forgive my ignorance, I am not an astronomer, just an interested layman.

The further away a galaxy is from us, the faster it is moving away from us. But the galaxies we see exist in the distant past because it takes a long time for their light to reach us. The closer galaxies are, the more recent their light is, and the slower they move away from us. Doesn't this seem to indicate that the expansion of the universe is slowing down, because the more recent light from galaxies, appears to be moving away slower? Does this make sense?

Quote from Wikipedia:

Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts. This was interpreted to mean that all distant galaxies and clusters are receding away from our vantage point with an apparent velocity proportional to their distance: that is, the farther they are, the faster they move away from us, regardless of direction.


As a fellow layman, I'll give this my best shot.

The further away a galaxy is from us, the faster it is moving away from us. But the galaxies we see exist in the distant past because it takes a long time for their light to reach us. The closer galaxies are, the more recent their light is, and the slower they move away from us.

All true. Think of a single point explosion, at any given time, the object twice as far should be moving twice as fast, cause they both started in the same place at the same time, so twice as fast moves twice s far. There are, complications - with the explosion there's air resistance, with the universe well, I'll get to that.

Doesn't this seem to indicate that the expansion of the universe is slowing down, because the more recent light from galaxies, appears to be moving away slower? Does this make sense?

OK, it sounds like you have an incorrect assumption there. Expansion means the far away is moving away faster and the closer is moving away slower, that's true with accelerating or decelerating expansion.

What they look for when studying expansion is measuring the precise speed and comparing that to distance. In steady expansion, you expect to see the galaxy 8 billion light years away to be apparently moving away twice as fast as the galaxy 4 billion light years away. (I say apparently cause we can only observe the relative velocity from 8 billion years in the past).

So, whether the galaxy 8 billion light years is traveling slightly more or slightly less than twice as fast away from us as the galaxy 4 billion light years away - that's what tells us about acceleration vs deceleration.

Imagine a car driving 60 MPH, and the driver puts the ever so slightest pressure on one of the pedals, but you don't know which one, and similarly the car driving 30 MPH does the same, but we can only see how fast the 60 MPH car was driving 6 minutes ago and we see how fast the car driving 30 was driving 3 minutes ago - that gives the 30 MPH car more time to apply the acceleration or deceleration. It's that time differential that gives us the information.

So, like the cars above, the galaxy 4 billion light years away has been in the accelerating expansion of space longer than the one 8 billion years ago and that's what tells us that the acceleration of expansion is happening, the nearer galaxies are moving a bit faster away from us than they would in a steady expansion or a gravitationally slowed expansion. (edited to make a bit more clear)

The newer light is moving slower. The older light moving faster. So from this it seems that the universe was expanding faster in the past?

light doesn't move slower or faster. it red-shifts when objects are moving away from each other. The faster they move away the greater the red-shift. That's one way relative velocity can be measured.


Brilliant question and great guess. The farther we see in space, the farther we see back in time; and the farther we see back in time, the faster was acceleration. And vice versa. In fact, our local universe--Laniakea--is contracting. Laniakea is 500 million light years across. It suggests that the expansion stopped 500 million years ago and contraction began. Galaxies farther than 500 light years away we still see redshifted because more than 500 million years ago they still were running away. Today they are blueshifted but humans will see this blueshift only in very distant future.


How fast is the universe expanding?

Artists illustration of the expansion of the Universe. Credit: NASA, Goddard Space Flight Center

The Universe is expanding, but how quickly is it expanding? How far away is everything getting from everything else? And how do we know any of this anyway?

When astronomers talk about the expansion of the Universe, they usually express it in terms of the Hubble parameter. First introduced by Edwin Hubble when he demonstrated that more distant galaxies are moving away from us faster than closer ones.The best measurements for this parameter gives a value of about 68 km/s per megaparsec.

Let's recap. Hubble. Universe. Galaxies. Leaving. Further means faster. And then I said something that sounded like "blah blah Lando blah blah Kessel Run 68 km/s per megaparsec". Which translates to if you have a galaxy 1 megaparsec away, that's 3.3 million light years for those of you who haven't seen Star Wars, it would be expanding away from us at a speed of 68 km/s. So, 1 megaparsec in distance means it's racing away at 68 km/s.

This is all because space is expanding everywhere in all places, and as a result distant galaxies appear to be expanding away from us faster than closer ones. There's just more "space" to expand between us and them in the first place. Even better, our Universe was much more dense in the past, as a result the Hubble parameter hasn't always had the same value.

There are two things affecting the Hubble parameter: dark energy, working to drive the Universe outwards, and matter, dark and regular flavor trying to hold it together. Pro tip: The matter side of this fight is currently losing.

Earlier in the Universe, when the Hubble parameter was smaller, matter had a stronger influence due to its higher overall density. Today dark energy is dominant, thus the Hubble parameter is larger, and this is why we talk about the Universe not only expanding but accelerating.

Our cosmos expands at about the rate at which space is expanding, and the speed at which objects expand away from us depends upon their distance. If you go far enough out, there is a distance at which objects are speeding away from us faster than the speed of light. As a result, it's suspected that receding galaxies will cross a type of cosmological event horizon, where any evidence of their existence, not even light, would ever be able to reach us, no matter how far into the future you went.

What do you think? Is there anything out there past that cosmological event horizon line waiting to surprise us?

Expansion of the Universe. Credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.

The future of humanity: can we avert disaster?

Climate change and artificial intelligence pose substantial — and possibly existential — problems for humanity to solve. Can we?

  • Just by living our day-to-day lives, we are walking into a disaster.
  • Can humanity wake up to avert disaster?
  • Perhaps COVID was the wake-up call we all needed.

Does humanity have a chance for a better future, or are we just unable to stop ourselves from driving off a cliff? This was the question that came to me as I participated in a conference entitled The Future of Humanity hosted by Marcelo's Institute for Cross-Disciplinary Engagement. The conference hosted an array of remarkable speakers, some of whom were hopeful about our chances and some less so. But when it came to the dangers facing our project of civilization, two themes appeared in almost everyone's talks.

And here's the key aspect that unifies those dangers: we are doing it to ourselves.


The Hubble Constant Surd

When talks ended for the day, many attendees piled into a van bound for the hotel. We drove past palm trees with the ocean on the right and the Santa Ynez Mountains to the distant left. Wendy Freedman, a decorated Hubble constant veteran, perched in the second row. A thin, calm woman of 62, Freedman led the team that made the first measurement of H0 to within 10% accuracy, arriving at a result of 72 in 2001.

The driver, a young, bearded Californian, heard about the Hubble trouble and the issue of what to call it. Instead of tension, problem or crisis, he suggested “surd,” meaning nonsensical or irrational. The Hubble constant surd.

Freedman, however, seemed less giddy than the average conferencegoer about the apparent discrepancy and wasn’t ready to call it real. “We have more work to do,” she said quietly, almost mouthing the words.

Freedman spent decades improving H0 measurements using the cosmic distance ladder method. For a long time, she calibrated her ladder’s rungs using cepheid stars — the same pulsating stars of known brightness that SH0ES also uses as “standard candles” in its cosmic distance ladder. But she worries about unknown sources of error. “She knows where all the skeletons are buried,” said Barry Madore, Freedman’s white-whiskered husband and close collaborator, who sat up front next to the driver.

Freedman said that’s why she, Madore and their Carnegie-Chicago Hubble Program (CCHP) set out several years ago to use “tip of the red giant branch” stars (TRGBs) to calibrate a new cosmic distance ladder. TRGBs are what stars like our sun briefly turn into at the end of their lives. Bloated and red, they grow brighter and brighter until they reach a characteristic peak brightness caused by the sudden igniting of helium in their cores. Freedman, Madore and Myung Gyoon Lee first pointed out in 1993 that these peaking red giants can serve as standard candles. Now Freedman had put them to work. As we unloaded from the van, I asked her about her scheduled talk. “It’s the second talk after lunch tomorrow,” she said.

“Be there,” said Madore, with a gleam in his eye, as we parted ways.

When I got to my hotel room and checked Twitter, I found that everything had changed. Freedman, Madore and their CCHP team’s paper had just dropped. Using tip-of-the-red-giant-branch stars, they’d pegged the Hubble constant at 69.8 — notably short of SH0ES’ 74.0 measurement using cepheids and H0LiCOW’s 73.3 from quasars, and more than halfway to Planck’s 67.4 prediction. “The Universe is just messing with us at this point, right?” one astrophysicist tweeted. Things were getting surd.

Dan Scolnic, a bespectacled young member of SH0ES based at Duke University, said that he, Riess and two other team members had gotten together, “trying to figure out what was in the paper. Adam and I then went out to dinner and we were pretty perplexed, because in what we had seen up to this point, the cepheids and TRGBs were in really good agreement.”

They soon homed in on the key change in the paper: a new way of measuring the effects of dust when gauging the intrinsic brightness of TRGBs — the first rung of the cosmic distance ladder. “We had a bunch of questions about this new method,” Scolnic said. Like other participants scattered throughout the Best Western Plus, they eagerly awaited Freedman’s talk the next day. Scolnic tweeted, “Tomorrow is going to be interesting.”


Mysterious Dark Energy Confirmed By New Method

Billions of years ago, theuniverse was crowded with tight-knit clusters of galaxies. Then, a partycrasher got the upper hand. This mysterious force now called dark energy hassince been expanding the universe at an increasing pace.

New measurements of this acceleratingexpansion, which drives galaxies away from one another on large scales butso far shows no effects on small scales (such as within a galaxy), providedetails about the nature of the unseen and unknown dark energy that is at work.

The results, announced todayat a news conference organized by NASA, reveal a decrease in the mass of galaxyclusters in more recent times, which would be a consequence of this hasteningand ripping force that some think could eventuallytear apart even star systems, planets and eventually the very moleculeswe're made of.

"If there were any doubts10 years after the initial discovery that the universe was speeding up, thisshould really dispel them," said Michael Turner of the University of Chicago's Department of Astronomy and Astrophysics, who was not involved in thecurrent study.

In addition, the new results,which relied on the Chandra X-ray Observatory, suggest darkenergy takes the form of what Einstein called the cosmological constant ? aterm in Einstein's general relativity that represents the possibility of emptyspace having a density and pressure associated with it.

If darkenergy is indeed some kind of repulsive force that is linked with"nothing," and the density of dark energy stays the same over time,astrophysicists say the expansion of the universe will continue to speed up. Sorather than galaxies mingling and merging, they will fly away from one another.

Andbillions of years from now, the scientists say, local superclusters of galaxieswill also disintegrate and all other galaxies will ultimately disappear fromthe Milky Way's view.

"Wedon't really have a clue why the universe is speeding up. We have some ideas,but we really don't understand it," said Turner, who is credited withcoming up with the term "dark energy." "And so having yetanother method to study how that speed-up happened can only help us, can onlymake us more optimistic about eventually understanding what the dark energyis."

Dark energy was discovered in1998 by two teams of astronomers, who measured light coming from explodingstars called Type IA supernovae, known as "standard candles" fortheir consistent brightness. The striking result was that distant supernovaewere dimmer (farther away) than they would be in a universe that was slowingdown. The result suggested the expansion of the universe was accelerating. Andthe teams proposed something called dark energy could be driving thisacceleration. This acceleration, it is thought, began about 5 billion yearsago.

That was the first stand-aloneevidence to support the idea of darkenergy.

And whereas then the repulsive force could have been brushed off as the result of possible errors in the measurements, more and more independent detections have solidified dark energy's existence.

Astronomers estimate now thatout of the total mass-energy budget in the universe, about 74 to 76 percent isdark energy, 20 to 22 percent is dark matter and 4 percent or so is normalmatter that makes stars, planets and everything we see. And they know that some"force" is causing galaxies to fly away from one another, operatinglike antigravity.

Rather than using Type IA supernovae, the new study is based on observations of clusters of galaxies atdifferent time points in the history of the universe. Scientists say the newstudy marks the second stand-alone evidence for the existence of dark energy.?

"This is surely the bestjob that anyone has been able to do so far in using clusters to measure how theuniverse has gotten clumpy over time," said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Massachusetts. Kirshner, who was not involved inthe current study, was on one of the teams that first discovered dark energy.

A team led by Alexey Vikhlininof the Smithsonian Astrophysical Observatory in Cambridge, Mass., used theChandra X-ray Observatory to measure the hot gas in dozens of galaxy clusters,which are the largest collapsed objects in the universe.

Some of these clusters arerelatively nearby and others are more than halfway across the universe.Basically, the team was looking at X-rays emitted from this hot gas as it fellinto areas chock-fullof dark matter, or the mysterious material thought to act as scaffoldingonto which galaxies mature. The X-rays can be converted into mass for a givencluster at a given point in time (depending on the age of the cluster).

"It's like there's atug-of-war going on between the dark matter trying to slow things down andclump things and the dark energy trying to speed things up and eventuallymaking it hard for the galaxies or the dark matter to cluster," Kirshnertold SPACE.com.

When astronomers look fartheracross the cosmos, they are looking back in time. And in fact, the results showan increase in the mass of the galaxy clusters further back in time,which supports the idea that dark energy started to win out in the tug of warat some point in the universe's history. Astronomers are not certain on thetiming of the change from an expanding universe to one whose expansion isspeeding up.

With dark energy taking over,it would be more difficult for objects such as galaxies to get together andform clusters as space is being stretched. So astronomers would expect to see aslowdown of the growth of galaxy clusters in a dark-energy-dominated universe.

"This result could bedescribed as 'arrested development of the universe'," Vikhlinin said."Whatever is forcing the expansion of the universe to speed up is alsoforcing its development to slow down."

Future of dark energy

While the new galaxy-clusterresults strengthen the case for an accelerating universe, scientists have along trek before cracking the case of what dark energy is.

"I don't see us solvingthis in two or three years. We're going to have to bring to bear lots ofdifferent techniques to figure out what dark energy is," Turner saidduring a telephone interview. "This is a very big puzzle. This may be themost profound problem in all of science."

Scientists can continue tostudy the clustering of galaxies over time. Further study could show darkenergy doesn't take the form of the cosmological constant. For instance,another idea is that general relativity falls apart on larger scales.

And perhaps dark energy isstirring up trouble in another way, not just speeding up the expansion of theuniverse. "One of the areas where we think we might get really lucky isthat maybe there will be some other manifestation [of dark energy], but wedon't have any yet," Turner said. "And there's so much of this stuffin the universe it's hard to believe there isn't another manifestation. Butright now the only thing we know that dark energy is doing is causing theuniverse to speed up."


The Day Edwin Hubble Realized Our Universe Was Expanding

This year marks the 90th anniversary of a mind-boggling discovery: that the universe is expanding.

The discovery was spearheaded by Edwin Hubble, for whom the orbiting Hubble Space Telescope is named. As an astronomer at Mount Wilson Observatory in Los Angeles, Hubble had access to the most cutting-edge equipment of the day, particularly the 100-inch (2.5 meters) Hooker telescope. The telescope, built in 1917, was the largest on Earth until 1949.

Since 1919, Hubble had been discovering new galaxies from the observatory, according to the Carnegie Institution for Science. In 1923, he developed a method of measuring the distance between a far-flung galaxy and the Milky Way, which involved calculating the actual brightness of stars in another galaxy and then comparing that value with how bright they appeared from Earth. [11 Fascinating Facts About Our Milky Way Galaxy]

This work led to another revelation. According to the Carnegie Institution, Hubble also knew about the work of an earlier astronomer, Vesto Melvin Slipher, who had figured out that he could measure how fast a galaxy was moving toward or away from the Milky Way by looking for changes in the wavelengths of light coming from that galaxy. The measurement is called the Doppler shift, and the principle is the same as the pitch change that seems to happen as an ambulance siren approaches, blares by, and recedes, except with light instead of sound. In the case of light, wavelengths emitted by an object moving toward a stationary observer appear more frequent, and thus bluer. Wavelengths emitted by an receding object appear less frequent, and thus redder.

Armed with information about the distance of other galaxies and their Doppler shift, Hubble and his colleagues published a paper in 1929 that would change astronomy. The paper, "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae," demonstrated that the galaxies visible from the Milky Way all seemed to be speeding away. (On Jan. 17, 1929, the paper was "communicated" to the National Academy of Sciences.)

What Hubble and his co-authors had observed was the very expansion of the universe itself. To use a famous analogy, the galaxies are like raisins in the bread dough of the universe. As the dough rises, all of the raisins move farther apart, but they're all still stuck in the same dough. The discovery enabled the calculation of the age of the universe: about 13.7 billion years old.

Ninety years after the Hubble team reported its findings, scientists are still trying to understand how this expansion works. Last year, using the telescope named for Hubble, astronomers reported that the expansion is faster than expected &mdash 73 kilometers per second per megaparsec, to be precise. A megaparsec is 3.3 million light-years, so this measurement means that for every 3.3 million light-years from Earth, a galaxy appears to be receding at 73 kilometers per second faster.

A few months later, the same researchers found that more distant reaches of the universe seem to be expanding less quickly, at 67 kilometers per second per megaparsec. The discrepancies suggest that something &mdash maybe dark energy or dark matter &mdash is influencing the universe's expansion in ways not yet understood.


Planets, planets everywhere

The first visible-light image of a planet orbiting another star was released by Hubble researchers in 2008. In this image, light from the star Formalhaut (shown as a white dot at center) has been blocked out, revealing a vast disk of dust. Embedded in that disk is Formalhaut b, a planet with three times the mass of Jupiter. Successive images show the planet in 2004 and 2006. NASA

In July 1994, just seven months after the first shuttle servicing mission, fragments of a comet torn apart by Jupiter's gravity slammed into the giant planet's atmosphere, blasting world-size blemishes in the cloud tops that were easily visible to amateur and professional astronomers alike.

But the clearest, most spectacular views came from the Hubble Space Telescope, a powerful demonstration of the observatory's ability to provide flyby-class views of other planets in Earth's solar system.

Hubble has been used to track Venusian clouds and dust storms on Mars, to study the churning atmospheres of Jupiter and Saturn, to monitor Saturn's rings and auroral displays on both planets and to keep tabs on Uranus and Neptune and their many moons. More recently, Hubble has been extensively used to map the moons of Pluto and help find post-flyby targets in the remote Kuiper Belt for NASA's Pluto-bound New Horizons spacecraft.

Getting spectacular images of Earth's neighbors was not a surprise. But actually imaging a planet orbiting another star -- a feat Hubble achieved in November 2008 -- and spectroscopically examining the atmospheres of several other extra-solar planets, are considered major achievements.

"When Hubble was launched, we didn't even have evidence there were planets around other stars," Riess said. "Not only have those been found, Hubble has helped characterize those. It's truly remarkable."


New method may resolve difficulty in measuring universe's expansion

Artist's impression of the explosion and burst of gravitational waves emitted when a pair of superdense neutron stars collide. New observations with radio telescopes show that such events can be used to measure the expansion rate of the Universe. Credit: NRAO/AUI/NSF

Astronomers using National Science Foundation (NSF) radio telescopes have demonstrated how a combination of gravitational-wave and radio observations, along with theoretical modeling, can turn the mergers of pairs of neutron stars into a "cosmic ruler" capable of measuring the expansion of the Universe and resolving an outstanding question over its rate.

The astronomers used the NSF's Very Long Baseline Array (VLBA), the Karl G. Jansky Very Large Array (VLA) and the Robert C. Byrd Green Bank Telescope (GBT) to study the aftermath of the collision of two neutron stars that produced gravitational waves detected in 2017. This event offered a new way to measure the expansion rate of the Universe, known by scientists as the Hubble Constant. The expansion rate of the Universe can be used to determine its size and age, as well as serve as an essential tool for interpreting observations of objects elsewhere in the Universe.

Two leading methods of determining the Hubble Constant use the characteristics of the Cosmic Microwave Background, the leftover radiation from the Big Bang, or a specific type of supernova explosions, called Type Ia, in the distant Universe. However, these two methods give different results.

"The neutron star merger gives us a new way of measuring the Hubble Constant, and hopefully of resolving the problem," said Kunal Mooley, of the National Radio Astronomy Observatory (NRAO) and Caltech.

The technique is similar to that using the supernova explosions. Type Ia supernova explosions are thought to all have an intrinsic brightness which can be calculated based on the speed at which they brighten and then fade away. Measuring the brightness as seen from Earth then tells the distance to the supernova explosion. Measuring the Doppler shift of the light from the supernova's host galaxy indicates the speed at which the galaxy is receding from Earth. The speed divided by the distance yields the Hubble Constant. To get an accurate figure, many such measurements must be made at different distances.

When two massive neutron stars collide, they produce an explosion and a burst of gravitational waves. The shape of the gravitational-wave signal tells scientists how "bright" that burst of gravitational waves was. Measuring the "brightness," or intensity of the gravitational waves as received at Earth can yield the distance.

Radio observations of a jet of material ejected in the aftermath of the neutron-star merger were key to allowing astronomers to determine the orientation of the orbital plane of the stars prior to their merger, and thus the "brightness" of the gravitational waves emitted in the direction of Earth. This can make such events an important new tool for measuring the expansion rate of the Universe. Credit: Sophia Dagnello, NRAO/AUI/NSF

"This is a completely independent means of measurement that we hope can clarify what the true value of the Hubble Constant is," Mooley said.

However, there's a twist. The intensity of the gravitational waves varies with their orientation with respect to the orbital plane of the two neutron stars. The gravitational waves are stronger in the direction perpendicular to the orbital plane, and weaker if the orbital plane is edge-on as seen from Earth.

"In order to use the gravitational waves to measure the distance, we needed to know that orientation," said Adam Deller, of Swinburne University of Technology in Australia.

Over a period of months, the astronomers used the radio telescopes to measure the movement of a superfast jet of material ejected from the explosion. "We used these measurements along with detailed hydrodynamical simulations to determine the orientation angle, thus allowing use of the gravitational waves to determine the distance," said Ehud Nakar from Tel Aviv University.

The collision of two neutron stars (GW170817) flung out an extraordinary fireball of material and energy that is allowing a Princeton-led team of astrophysicists to calculate the Hubble constant, the speed of the universe's expansion. They used a super-high-resolution radio 'movie' (left) that they compared to a computer model (right). To generate their 'movie,' the science team combined data from enough radio telescopes spread over a large enough region to generate an image with such high resolution that if it were an optical camera, it could see individual hairs on someone's head 6 miles away. The movie emphasizes observations taken 75 days and 230 days after the merger. The middle panel shows the radio afterglow light curve. Credit: Ore Gottlieb and Ehud Nakar, Tel Aviv University

This single measurement, of an event some 130 million light-years from Earth, is not yet sufficient to resolve the uncertainty, the scientists said, but the technique now can be applied to future neutron-star mergers detected with gravitational waves.

"We think that 15 more such events that can be observed both with gravitational waves and in great detail with radio telescopes, may be able to solve the problem," said Kenta Hotokezaka, of Princeton University. "This would be an important advance in our understanding of one of the most important aspects of the Universe," he added.

The international scientific team led by Hotokezaka is reporting its results in the journal Nature Astronomy.


Inconstant Dark Energy

Ever since the surprise discovery in 1998 that the expansion of the universe is accelerating, cosmologists have included a repulsive dark energy in their model of cosmic evolution. But its nature remains a mystery. The simplest possibility is that dark energy is the “cosmological constant” — the energy of space itself, with a constant density everywhere. But what if the amount of dark energy in the universe isn’t constant?

An extra dose of dark energy in the early universe, dubbed early dark energy, could reconcile the conflicting values of the Hubble constant. The outward pressure of this early dark energy would have sped up the universe’s expansion. “The tricky part is that [early dark energy] can’t really stick around it has to go away quickly,” said Lisa Randall, a particle physicist and cosmologist at Harvard.