Decreasing rate of Earth's rotation: where does the power go?

Decreasing rate of Earth's rotation: where does the power go?

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I understand from Wikipedia that, "The length of the day, which has increased over the long term of Earth's history due to tidal effects,… ". If the angular velocity of the earth is decreasing then the stored rotational energy is decreasing. Where does the power/energy go?

You're correct in assuming the net angular momentum of the system in question here will remain constant. The Moon's orbit around Earth is responsible for the slowing of Earth's rotation. This effect is extremely small.

The decrease in Earth's angular momentum is transferred to the moon, which resultantly sees it's orbit accelerate. This acceleration also causes the moon to move further and further from the Earth. This trend will continue until they reach a common speed.

Where does the power/energy go?

It goes into heating the Earth and the Moon. That heat in turn spreads out into the universe.

While the Earth-Moon system comes very close to conserving angular momentum, it does not conserve mechanical energy. In fact, that angular momentum transfers from the Earth's rotation to the Moon's orbit means that the total mechanical energy of the Earth-Moon system is necessarily reduced. Mechanical energy is only conserved in isolated, non-dissipative systems. The tides in the Earth's oceans, in the Earth as a whole, and in the Moon as a whole means that the Earth-Moon system is dissipative.

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Ask Ethan: Will Earth’s Temperature Start Decreasing Over The Next 20,000 Years?

Although our planet is thought to have had about a 2:1 ratio of oceans to continents throughout its . [+] history, there was a period from about 2.4 to 2.1 billion years ago where the surface was 100% covered in ice: a Snowball Earth scenario. Could our planet, despite global warming, actually become cooler over the next 20,000 years?

According to our best understanding of Earth’s climate, the global average temperature has increased significantly over the past

140 years: the amount of time for which a reliable, direct temperature record exists. It’s widely accepted that the driving force behind this increase is the human-caused emission of greenhouse gases such as CO2, which has increased in atmospheric concentration by about 50% from the pre-industrial levels that were present early in the 1700s. But humans aren’t the only entities that affect Earth’s climate there are natural variations that occur in the Earth-Sun system. Will they cause Earth’s temperature to decrease in the relatively near future? That’s what Ian Graham wants to know, as he writes in to ask:

“I'm trying to get my head around the Earth's axial tilt and the ramifications of the current 23.5 degree increase/decrease, and trying to understand Milankovitch's theory. If the Perihelion is increasing and the earth warms as a result, ignoring the greenhouse effects of humans, what is the effect of both the Perihelion increase and the movement of earth away from the Sun? My thought is the Earth's global temp should decrease over the next 20,000 years.”

There’s a lot to unpack here, so let’s start at the beginning: with Milankovitch himself.

The Earth in orbit around the Sun, with its rotational axis shown. All worlds in our solar system . [+] have seasons determined by either their axial tilt, the ellipticity of their orbits, or a combination of both. Although the axial tilt dominates Earth's seasons today, this may not always be the case.

Wikimedia commons user Tauʻolunga

Back in the early 1900s, Serbian astrophysicist Milutin Milankovitch decided to work on a puzzle that no one else had successfully solved: linking the physics that governed the Solar System with the theory of Earth's climate. As the Earth orbits the Sun, you'll barely notice any year-to-year changes, as they're relatively minuscule. Sure, the phases of the Moon shift, the exact date and time of equinoxes and solstices vary, and timekeeping requires the regular insertion of leap days to keep the seasons aligned with our calendar.

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While Newton’s law of gravitation and Kepler’s laws of planetary motion are relatively simple, however, anything more complex than the simplest system imaginable can lead to incredibly elaborate orbital complications. In the case of the Earth, it’s affected by:

  • the fact that it rotates on its axis,
  • it moves in an ellipse, rather than a circle, around the Sun,
  • it has a large, natural satellite: the Moon,
  • which in turn orbits the Earth tidally locked, inclined at an angle to Earth’s orbit and axial rotation, and in a quite eccentric ellipse,
  • and the small (but not completely negligible) gravitational influence of the other bodies in our Solar System.

All of these effects interplay with one another to determine the long-term evolution of Earth’s orbit.

When the Earth's north pole is maximally tilted away from the Sun, it's maximally tilted towards the . [+] full Moon, on the opposite side of the Earth, while when your hemisphere of the Earth is maximally tilted towards the Sun, it's maximally tilted away from the full Moon. The Moon stabilizes our orbit but also slows the Earth's rotation, with both the Moon and Sun as well as the other planets all playing roles in the long-term evolution of Earth's rotation, axial tilt, and orbital parameters.

National Astronomical Observatory ROZHEN

There are a few important rules at play. One is the law of gravitation, and the fact that these aren’t point-like objects we’re talking about, but rather spheroids: physical objects of a real, finite size and with intrinsic angular momentum to them. That angular momentum, for each object in our Solar System — and particularly for the Earth, Moon, and Sun — is split up into the spin of each body, or its rotational motion, and its orbital angular momentum, or its revolutionary motion. (Yes, even the Sun doesn’t remain stationary, but rather makes its own “wobbly” motion due to the gravitational influence of the other bodies in the Solar System.)

What Milankovitch found, perhaps surprisingly to some, is that these effects all add up to cause three major long-term variations, arising from the interactions of these Solar System bodies.

  1. Precession, or the fact that the direction that Earth’s axis points rotates over time.
  2. Axial tilt, which changes ever so slightly from its current 23.5° over time.
  3. Eccentricity, or how circular vs. elliptical Earth’s orbit is.

Although there are other effects, they’re all minor compared to these three major ones. Let’s look at them individually.

Earth's rotational axis will precess over time due to two combined effects: axial precession (shown . [+] here) and apsidal precession, as its elliptical orbit also precesses. The combined effects, which have

112,000 year periods, respectively, result in a total precessional period of closer to

1.) Precession. This one is actually pretty straightforward: the Earth spins on its axis, which is inclined at 23.5° with respect to our revolutionary path around the Sun. When our axis is pointed perfectly perpendicular to the line connecting the Earth to the Sun, we experience equinoxes when the axis is pointed along the Earth-Sun line, we experience solstices. Although the timing of both equinoxes and solstices would change over time, astronomically, the insertion of leap days keeps the equinoxes centered around March 21 and September 23, with the solstices occurring around December 21 and June 21.

But the physical direction that our axis point does, in fact, change over time. Right now Polaris is our “north star” because our axis points towards it to within 1°, which is remarkable but unusual for a bright star. Over long periods time, the direction that Earth’s rotational axis points will make a complete circle, as two effects both come into play:

  • our axial precession, which is Earth’s “wobble” relative to the stars, largely due to the Moon and Sun,
  • and our apsidal precession, which is how Earth’s ellipse “wobbles” as we orbit the Sun, primarily due to Jupiter’s and Saturn’s influences.

Today, in the year 2020, Polaris lies extremely close to the exact north celestial Pole. The red . [+] circle traces out the direction that Earth's axis will point along over time, indicating which star will best serve as a pole star in both the far future and the distant past. Vega, the brightest star in this vicinity, will be our pole star in a little over 13000 years.

Wikimedia Commons user Tauʻolunga

Axial precession causes Earth to make a full 360° turn on its axis every 25,771 years, while the apsidal precession leads to an additional 360° turn (in the same direction) every

112,000 years or so. For an observer on Earth, if we could live that long, we’d see the pole stars change in a periodic fashion every 23,000 years or so, as these effects combine in an additive fashion. Thousands of years ago, the star Kochab (the brightest star in the Little Dipper’s bowl) was where our North Pole pointed thousands of years from now, it will point at Vega, one of the brightest stars in the sky, 13,000 years in the future.

The main effect of this precession on temperature is seasonal, however, and has no long-term effect on an annual basis. Because the South Pole points towards the Sun close to the December solstice, orbital perihelion aligns with its summer and aphelion is close to its winter, resulting in colder winters and hotter summers compared to the Northern Hemisphere. This will change over time with a

23,000 year period, but presents no long-term, overall temperature variations.

41,000 years, Earth's axial tilt will vary from 22.1 degrees to 24.5 degrees . [+] and back. Right now, our tilt of 23.5 degrees is slowly decreasing from its maximum, which was reached just under 11,000 years ago, to its minimum, which it will achieve a little less than 10,000 years from now.

2.) Axial tilt. At present, the Earth rotates on its axis at an angle of 23.5°, and that axial tilt plays a more significant role than even how close or far we are from the Sun in determining our seasons. When the Sun’s rays are more direct on our portion of the Earth, we receive more energy from the Sun when they’re more indirect (incident at a lower angle and passing through more of our atmosphere), we receive less energy. Over the course of a year and averaged over the whole planet, our axial tilt doesn’t substantially affect how much total energy the Earth receives.

But our axial tilt does vary somewhat over long periods of time: from a minimum of 22.1° to a maximum of 24.5°, oscillating from its minimum to maximum and back to minimum again approximately every

41,000 years. Our Moon is primarily responsible for stabilizing our axial tilt the tilt of Mars is comparable to that of Earth, but Mars’s variations are about 10 times as great, because it lacks a large, massive moon to keep these axial tilt variations small.

Earth rotates on its axis, but its axial rotation varies by less than 2.5 degrees over time owing to . [+] the presence of a large, massive Moon. Mars, which has a similar axial tilt to Earth at present, sees variations in its tilt that are approximately a factor of 10 greater than Earth's due to the lack of such a moon.

Although the total energy received by our planet — and hence, Earth’s total temperature — isn’t affected by our axial tilt, the energy received as a function of latitude is very sensitive to it. When our axial tilt is lower, a greater percentage of the energy received by Earth is concentrated towards equatorial latitudes, while when it’s greater, less energy is received at the equator and more is incident on the poles. As a result, larger axial tilts favor the retreat of glaciers and polar ice sheets, while smaller axial tilts generally favor their growth.

Right now, our axial tilt is about midway between these two extremes, and in the process of decreasing. Our axial tilt last reached its maximum value nearly 11,000 years ago, corresponding to the end of our last glacial maximum, with our next minimum approaching in a little under 10,000 years. If natural variations were dominant, we’d expect the next

20,000 years to favor the growth of ice sheets. As NASA’s website says:

“As obliquity decreases, it gradually helps make our seasons milder, resulting in increasingly warmer winters, and cooler summers that gradually, over time, allow snow and ice at high latitudes to build up into large ice sheets. As ice cover increases, it reflects more of the Sun’s energy back into space, promoting even further cooling.”

This, very likely, is where the notion that Earth should start cooling again comes from.

Variations in the eccentricity of the ellipse that Earth traces out around the Sun occur in

100,000 . [+] year intervals, with maximal changes occurring over a period of every four cycles: with

400,000 year periods. The changes in orbit shape are the only one of the major Milankovitch cycles that change the total amount of solar radiation reaching Earth.

3.) Eccentricity. This effect, of all the effects caused by the dynamics experienced by Earth in the Solar System — gravitational forces, tides, angular momentum exchange, etc. — is the only one that changes the total amount of solar energy received by the Earth on an annual basis. Due largely to the gravitational tug of the gas giants, the eccentricity of Earth’s orbit (or how elongated its ellipse is, e, which is 0 for a perfect circle and approached 1 for an extremely long, skinny ellipse) varies in two ways:

  • with a periodicity on 100,000 year timescales, going from almost-perfectly circular orbits (e = 0) to near-maximum ellipticity,
  • and with additional slight magnifications every 400,000 years, leading to Earth’s orbit achieving its maximum ellipticities of all (e = 0.07).

Earth, right now, has a relatively small eccentricity: 0.017, which is close to the minimum value. Our closest approach to the Sun, perihelion, is only 3.4% closer than our farthest position, aphelion, and we receive just 7% more radiation from the Sun in that configuration. On the other hand, when our eccentricity is maximized, perihelion and aphelion differ by thrice that amount, with the difference in radiation received at perihelion vs. aphelion rising to 23%.

The orbits of the planets in the inner solar system aren't exactly circular, but they're quite . [+] close, with Mercury and Mars having the biggest departures and the greatest ellipticities. While Mars's orbital eccentricity, at 0.09, is much larger than Earth's at present (at 0.017), Earth's eccentricity can achieve a maximum of 0.07, rivaling Mars and potentially causing our seasons to be dominated by orbital position, rather than axial tilt, just like Mars.

When our orbit is more eccentric, our seasons can even become dominated by our orbital position, rather than our axial tilt. However, that’s unlikely to happen anytime soon. Right now, our eccentricity is close to the minimum, and is decreasing further: towards zero. And in general, higher eccentricity — a more elliptical orbit as compared to a more circular one — means a greater amount of solar radiation received by Earth over the course of a year.

  • The maximum amount of radiation Earth can receive occurs when our eccentricity is maximized, and we can call that “100%” of maximum.
  • For a perfectly circular orbit, we’d still receive 99.75% of that maximum amount.
  • For where we are right now in our orbit, we receive almost that same value: 99.764%, which is presently decreasing towards that 99.75% value.

There is a slight decrease that’s in progress, but it’s so minuscule that it’s practically negligible — as are all of these cumulative effects — in comparison to the enormous changes brought on by the human-caused greenhouse gas contribution to global temperature.

The global surface average temperature for the years where such records reliably and directly exist: . [+] 1880-2019 (at present). The zero line represents the long-term average temperature for the whole planet blue and red bars show the difference above or below average for each year. The warming, on average, is by 0.07 C per decade, but has accelerated, warming at an average of 0.18 C since 1981.

Looking at the effects of Earth’s orbital changes quantitatively — including all three effects of precession, axial tilt, and elliptical eccentricity — so clearly illustrates the incredible conundrum facing humanity today. Because of the increased concentration of greenhouse gases, Earth’s average global temperature has increased by approximately 0.98°C (1.76°F) since 1880: an increase of approximately 0.33% in the average energy retained by the Earth. This human-caused effect has, by far, the dominant impact on Earth’s climate of all of these factors.

The increased energy retention due to atmospheric changes dwarfs the coming 0.014% decrease in received energy arising from the change in our ellipse’s shape, and overwhelms the axial tilt changes, which redistribute only an extra 0.0002% of the polar energy towards the equator with each passing year. It even dwarfs the 0.08% variation that occurs coincident with the 11-year sunspot cycle. Unless we address the human factors which currently dominate the changing climate of Earth, these natural factors — important and real though they may be — will be overwhelmed by our own recklessness.

Earth's day lengthens by two milliseconds a century, astronomers find

There may never be enough hours in the day to get everything done, but at least the forces of nature are conspiring to help out.

Astronomers who compiled nearly 3,000 years of celestial records have found that with every passing century, the day on Earth lengthens by two milliseconds as the planet’s rotation gradually winds down.

The split second gained since the first world war may not seem much, but the time it takes for a sunbeam to travel 600km towards Earth can cost an Olympic gold medal, as the American Tim McKee found out when he lost to Sweden’s Gunnar Larsson in 1972.

For those holding out for a whole extra hour a day, be prepared for a long wait. Barring any change in the rate of slowing down, an Earth day will not last 25 hours for about two million centuries more.

Researchers at Durham University and the UK’s Nautical Almanac Office gathered historical accounts of eclipses and other celestial events from 720BC to 2015. The oldest records came from Babylonian clay tablets written in cuneiform, with more added from ancient Greek texts, such as Ptolemy’s 2nd century Almagest, and scripts from China, medieval Europe and the Arab dominions.

The ancient records captured the times and places that people witnessed various stages of solar and lunar eclipses, while documents from 1600AD onwards described lunar occultations, when the moon passed in front of particular stars and blocked them from view.

To find out how the Earth’s rotation has varied over the 2,735-year-long period, the researchers compared the historical records with a computer model that calculated where and when people would have seen past events if Earth’s spin had remained constant.

“Even though the observations are crude, we can see a consistent discrepancy between the calculations and where and when the eclipses were actually seen,” said Leslie Morrison, an astronomer on the team. “It means the Earth has been varying in its state of rotation.”

The Earth formed from a spinning cloud of dust and gas 4.5bn years ago, but it is thought to have received an extra rotational kick when a Mars-sized object crashed into the young planet and knocked off the material that became the moon. In that cataclysmic event, a day on Earth may have leapt from six hours to 24 hours.

But astronomers have long known that Earth’s spin is slowing down. The main braking effect comes from tides caused by the moon’s gravity. “The heaping up of water drags on the Earth as it spins underneath,” said Morrison. As Earth’s rotation slows, the moon’s orbit grows by about 4cm a year.

Tidal braking is not the only force at work though. The astronomers found that Earth’s spin would have slowed down even more had it not been for a counteracting process. Since the end of the most recent ice age, land masses that were once buried under slabs of frozen water have been unloaded and sprung back into place. The shift caused the Earth to be less oblate – or squished – on its axis. And just as a spinning ice skater speeds up when she pulls in her arms, so the Earth spins faster when its poles are less compressed.

Changes in the world’s sea levels and electromagnetic forces between Earth’s core and its rocky mantle had effects on Earth’s spin too, according to the scientists’ report in Proceedings of the Royal Society. The different forces seem to drive cycles in the Earth’s rotation spanning decades to centuries, with one cycle repeating every 1500 years.

“Geological processes occur on long time scales which makes direct observation of their evolution extremely difficult on human timescales,” said Jon Mound, a geophysicist at Leeds University who was not involved in the research. “This is a particular problem for phenomena such as the Earth’s rotation which don’t leave direct evidence in the geological record.”

“In many ways this is an amazing result that ties together a wide range of investigations at opposite ends of the scale of technological sophistication to determine to high precision an extremely small effect,” he added.

Decreasing rate of Earth's rotation: where does the power go? - Astronomy

So clearly the Moon influences the Earth in more than just an aesthetic way.

Think about this for class:

In what other ways does the moon affect the Earth and its inhabitants?

Tidal Evolution of Orbits

As the Moon's tidal forces raise bulges on the Earth, the Earth's rotation moves these bulges forward to lead the Moon:

Since the tidal bulge has a little extra mass associated with it, the moon feels a little extra pull forward in its orbit . (Note that this is the opposite of what is happening to Mars' moon Phobos, who leads the bulges on Mars and thus feels a backwards drag .)

So the moon is being accelerated forwards, as if it had a little rocket on its back firing away.

Think about what happens in an orbit if you accelerate forwards. You are adding energy and angular momentum to your orbit, so you move outwards into a larger orbit with a longer orbital period.

The Moon is slowly moving away from the Earth!

How fast? About 3-4 cm/yr. (How do we measure this?)

So the moon is moving away, gaining energy and angular momentum from the Earth. But a basic law of physics says that energy and angular momentum are conserved (if there are no external torques).

So if the Moon gains angular momentum, the Earth must lose angular momentum.

The Earth's rotation rate is gradually slowing!

How fast? About 0.0016 seconds/century, or roughly 1 second every 50,000 years.
So a "dinosaur day" was about 23.5 hours long, not 24 hours long.
(Extrapolation doesn't work too far back, but best current models suggest that when the Earth and Moon were closest several billion years ago, one day was about 4-5 hours long!)

Can this go on forever? When does this all end?

  • The Moon is moving away: tidal forces are decreasing Earth's bulge is lessening.
  • The Earth's rotation is slowing: bulge and Moon closer to alignment.

(What other familiar object is in a 1:1 synchronous rotation?)

At this point, with the bulges aligned with the Moon, no further orbit evolution will occur.

Earth's Rotation Is Mysteriously Slowing Down: Experts Predict Uptick In 2018 Earthquakes

A depiction of Earth's interior, showing the movement of molten rock, which makes up the mantle.

Scientists have found strong evidence that 2018 will see a big uptick in the number of large earthquakes globally. Earth's rotation, as with many things, is cyclical, slowing down by a few milliseconds per day then speeding up again.

You and I will never notice this very slight variation in the rotational speed of Earth. However, we will certainly notice the result, an increase in the number of severe earthquakes.

Geophysicists are able to measure the rotational speed of Earth extremely precisely, calculating slight variations on the order of milliseconds. Now, scientists believe a slowdown of the Earth's rotation is the link to an observed cyclical increase in earthquakes.

To start, the research team of geologists analyzed every earthquake to occur since 1900 at a magnitude above 7.0. They were looking for trends in the occurrence of large earthquakes. What they found is that roughly every 32 years there was an uptick in the number of significant earthquakes worldwide.

The team was puzzled as to the root cause of this cyclicity in earthquake rate. They compared it with a number of global historical datasets and found only one that showed a strong correlation with the uptick in earthquakes. That correlation was to the slowing down of Earth's rotation. Specifically, the team noted that around every 25-30 years Earth's rotation began to slow down and that slowdown happened just before the uptick in earthquakes. The slowing rotation historically has lasted for 5 years, with the last year triggering an increase in earthquakes.

To add an interesting twist to the story, 2017 was the 4th consecutive year that Earth's rotation has slowed. This is why the research team believes we can expect more earthquakes in 2018, it is the last of a 5-year slowdown in Earth's rotation.

What Is Causing Earth's Rotation To Slow Down?

As with many new findings in science, this story began with the data that supports the cyclical slowdown then speed up of Earth's rotation. The research team is then tasked with the "why" to explain this phenomenon. While scientists aren't exactly sure the mechanisms that produce this variation, there are a few hypotheses.

One hypothesis involves Earth's outer core, a liquid metal layer of the planet that circulates underneath the solid lower mantle. The thought is that the outer core can at times "stick" to the mantle, causing a disruption in its flow. This would alter Earth's magnetic field and produce a temporary hiccup in Earth's rotation.

Currently, the data only notes a striking correlation, but no causation. Hence, scientists are still unsure whether this change in Earth's rotation is the cause of an uptick in earthquakes.

While there's no direct link between the two, the trend over the past century suggests that 2018 will be an unusually active year for earthquakes. Typically, there will be 15-20 large earthquakes (M 7.0 or greater). However, during the noted uptick in earthquakes aligning to the 5th year of Earth's rotation slowdown there are on average 25-30 large earthquakes.

Earthquakes remain the most difficult natural disaster to predict. They tend to occur with little to no forewarning and can thus be incredibly destructive. Often times, geologists are limited to historical trends in data to predict the likelihood an earthquake will occur. This new research provides another dataset to inform communities about the near-term risks they face.

Liked the article? I highly recommend reading our in-house astrophysicist's thoughts on the subject.

Would the Earth's rotational movement be affected by the melting of the polar ice caps?

You are suggesting that the migration of fluid from the polar ice cap to the oceans will make the earth "shorter and fatter", thus slowing its spin rate? And that the resulting change in day length will affect agriculture?

Have you calculated how much change in day length would result from moving the [south] polar ice cap to the equator? Make some generous assumptions and see what you get.

As the Antarctic ice cap melts it will distribute it's mass equally over the oceans. The moment of inertia of the Earth will change as the mass of the ice is redistributed from a disk near the axis to a spherical shell over the oceans.

Conservation of angular momentum will slow the Earth. The day will lengthen by less than a second, but sundials will also slow down to fully compensate.

The main change that people and their economy will have is sea level rise, firstly because of the melt water, and secondly because of the thermal expansion of the water column happening at the same time due to the warming that melts the ice. The coastal regions and many major cities will be drowned long before anyone notices the solar days are very slightly longer. So yes, the Earth will slow, the days will lengthen, but you will certainly not notice it without an atomic clock.

What would happen if the earth's rotation was faster or slower?

Days & nights would be shorter or longer, and our weight would be less or more.


If it was faster then one full rotation would take less than 24 hrs., thus making days & nights shorter. Our weight would be less, because as the Earth would rotate faster, it would exert more centrifugal force on us. The resultant force of the Earth's gravity and the centrifugal force would be less as gravity would remain constant but centrifugal force would increase. There would also be a temperature change as each hemisphere (Eastern and Western) would get less time to warm up from the Sun's rays.

If it was slower then one full rotation would take more than 24 hrs., thus making days & nights longer. Our weight would be more, because as the Earth would rotate slower, it would exert less centrifugal force on us. The resultant force of the Earth's gravity and the centrifugal force would be more as gravity would remain constant but centrifugal force would decrease. There would also be a temperature change as each hemisphere (Eastern and Western) would get more time to warm up from the Sun's rays.

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Physics Check

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Phy 231

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Science check my answers please.

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Did We Just Find The Largest Rotating ‘Thing’ In The Universe?

Cosmic filaments are among the largest structures in the Universe, and they rotate. In a new study . [+] that stacked thousands of filaments together, they were observed to be rotating along their filamentary axis, with the average rotation speed approaching

AIP (Leibniz Institute for Astrophysics Potsdam)/A. Khalatyan/J. Fohlmeister

In our own cosmic backyard, everything we see spins, rotates, and revolves in some fashion or other. Our planet (and everything on it) spins about its axis, just like every planet and moon in the Solar System. The moons (including our own) revolve around their parent planet, while the planet-moon systems all revolve around the Sun. The Sun, in turn, like all of the hundreds of billions of stars in the galaxy, orbit around the galactic center, while the entire galaxy itself spins about the central bulge.

On the largest of cosmic scales, however, there’s no observed global rotation. The Universe, for whatever reason, doesn’t appear to have an overall spin or rotation to it, and doesn’t appear to be revolving around anything else. Similarly, the largest observed cosmic structures don’t appear to be spinning, rotating, or revolving around any other structures. But recently, a new study appears to be challenging that, claiming that enormous cosmic filaments — the strands of the cosmic web — appear to be rotating about the filamentary axis itself. This is weird, for sure, but can we explain it? Let’s find out.

Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and . [+] evolution, and continues to do so. Our entire observable Universe was approximately the size of a soccer ball some 13.8 billion years ago, but has expanded to be

46 billion light-years in radius today.

In order to make a prediction, we first have to set up the scenario that we expect, then put in the laws of physics, and evolve the system forward in time to see what we anticipate. We can go all the way back, theoretically, to the earliest stages of the Universe. At the start of the hot Big Bang, immediately following the end of cosmic inflation, the Universe is:

  • filled with matter, antimatter, dark matter, and radiation,
  • uniform and the same in all directions,
  • with the exception of slight density imperfections on the scale of 1-part-in-30,000,
  • and with additional tiny imperfections in the directionality of these fluctuations, the linear and rotational motions of these overdense and underdense regions, and similar imperfections in gravitational wave background that the Universe is born with.

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As the Universe expands, cools, and gravitates, a number of important steps occur, particularly on large cosmic scales.

The cold fluctuations (shown in blue) in the CMB are not inherently colder, but rather represent . [+] regions where there is a greater gravitational pull due to a greater density of matter, while the hot spots (in red) are only hotter because the radiation in that region lives in a shallower gravitational well. Over time, the overdense regions will be much more likely to grow into stars, galaxies and clusters, while the underdense regions will be less likely to do so. The gravitational density of the regions the light passes through as it travels can show up in the CMB as well, teaching us what these regions are truly like.


In particular, some things grow with time, other things decay with time, and still other things remain the same with time.

The density imperfections, for example, grow in a particular fashion: proportional to the ratio of the matter density to the radiation density. As the Universe expands and cools, both matter and radiation — made up of individual quanta — get less dense the number of particles remains the same while the volume increases, causing the density of both to drop. They don’t drop equally, however the amount of mass in every matter particle remains the same, but the amount of energy in every quantum of radiation drops. As the Universe expands, the wavelength of the light traveling through space stretches, bringing it to lower and lower energies.

As the radiation gets less energetic, the matter density rises relative to the radiation density, causing these density imperfections to grow. Over time, the initially overdense regions preferentially attract the surrounding matter, drawing it in, while the initially underdense regions preferentially give up their matter to the denser regions nearby. Over long enough timescales, this leads to the formation of molecular gas clouds, stars, galaxies, and even the entire cosmic web.

The growth of the cosmic web and the large-scale structure in the Universe, shown here with the . [+] expansion itself scaled out, results in the Universe becoming more clustered and clumpier as time goes on. Initially small density fluctuations will grow to form a cosmic web with great voids separating them, but what appear to be the largest wall-like and supercluster-like structures may not be true, bound structures after all.

Similarly, you can track the evolution of any initial rotational modes in a Universe that’s initially isotropic and homogeneous. Unlike the density imperfections, which grow, any initial spin or rotation will decay away as the Universe expands. Specifically, it decays as the scale of the Universe grows: the more the Universe expands, the less important angular momentum becomes. It should make sense, therefore, to anticipate that there won’t be any angular momentum — and hence, any spinning or rotation — on the largest cosmic scales.

At least, that’s true, but only up until a certain point. As long as your Universe, and the structures in it, continue to expand, these rotational or spinning modes will decay away. But there’s a rule that’s even more fundamental: the law of conservation of angular momentum. Just like a spinning figure skater can increase their rate of rotation by bringing their arms and legs in (or can decrease it by moving their arms and legs out), the rotation of large-scale structures will diminish so long as the structures expand, but once they get pulled in under their own gravity, that rotation speeds up again.

When a figure skater like Yuko Kawaguti (pictured here from 2010's Cup of Russia) spins with her . [+] limbs far from her body, her rotational speed (as measured by angular velocity, or the number of revolutions-per-minute) is lower than when she pulls her mass close to her axis of rotation. The conservation of angular momentum ensures that as she pulls her mass closer to the central axis of rotation, her angular velocity speeds up to compensate.

deerstop / Wikimedia Commons

Angular momentum, you see, is a combination of two different factors multiplied together.

  1. Moment of inertia, which you can think about as how your mass is distributed: close to the rotation axis is a small moment of inertia far away from the rotation axis is a large moment of inertia.
  2. Angular velocity, which you can think of as how quickly you make a complete revolution something like revolutions-per-minute is a measure of angular velocity.

Even in a Universe where your density imperfections are born only with a very slight amount of angular momentum, gravitational growth won’t be able to get rid of it, while gravitational collapse, which causes your mass distribution to get concentrated towards the center, ensures that your moment of inertia will eventually decrease dramatically. If your angular momentum stays the same while your moment of inertia goes down, your angular velocity must rise in response. As a result, the greater the amount of gravitational collapse a structure has undergone, the greater the amount we expect to see it spinning, rotating, or otherwise manifesting its angular momentum.

In isolation, any system, whether at rest or in motion, including angular motion, will be unable to . [+] change that motion without an outside force. In space, your options are limited, but even in the International Space Station, one component (like an astronaut) can push against another (like another astronaut) to change the individual component's motion.

NASA / International Space Station

But even that is only half of the story. Sure, we fully expect that the Universe is born with some angular momentum, and when these density imperfections grow, attract matter, and finally collapse under their own gravity, we expect to see them rotating — perhaps even quite substantially — in the end. However, even if the Universe were born with no angular momentum anywhere at all, it’s an inevitability that the structures that form on all cosmic scales (except, perhaps, the extreme largest ones of all) will start spinning, rotating, and even revolving around one another.

The reason for this is a physical phenomenon we’re all familiar with, but in a different context: tides. The reason planet Earth experiences tides is because the objects near it, like the Sun and the Moon, gravitationally attract the Earth. Specifically, however, they attract every point on the Earth, and they do so unequally. The points on the Earth that are closer to the Moon, for instance, get attracted a little bit more than the points that are farther away. Similarly, the points that are “north” or “south” of the imaginary line that connects Earth’s center to the Moon’s center will be attracted “downward” or “upward” correspondingly.

At every point along an object attracted by a single point mass, the force of gravity (Fg) is . [+] different. The average force, for the point at the center, defines how the object accelerates, meaning that the entire object accelerates as though it were subject to the same overall force. If we subtract that force out (Fr) from every point, the red arrows showcase the tidal forces experienced at various points along the object. These forces, if they get large enough, can distort and even tear individual objects apart.

Despite how easy this is to visualize for a round body like the Earth, the same process takes place between every two masses in the Universe that occupy any volume more substantial than a single point. These tidal forces, as objects move through space relative to one another, exerts what’s known as a torque: a force that causes objects to experience a greater acceleration on one part of it than other parts of it. In all but the most perfectly aligned cases — where all the torques cancel out, a tremendous and coincidental rarity — these tidal torques will cause an angular acceleration, leading to an increase in angular momentum.

“Hang on,” I can hear you objecting. “I thought you said that angular momentum was always conserved? So how can you create an angular acceleration, which increases your angular momentum, if angular momentum is something that can never be created or destroyed?”

It’s a good objection. What you have to remember, however, is that torques are just like forces in the sense that they obey their own versions of Newton’s laws. In particular, just like forces have directions, so do torques: they can cause something to rotate clockwise or counterclockwise about each of the three-dimensional axes that exist in our Universe. And just like every action has an equal an opposite reaction, whenever one object pulls on another to create a torque, that equal and opposite force will create a torque on that first object as well.

Many have tried to surpass the current land speed record by attaching rockets or other . [+] thrust-providing contraptions to their vehicles. When the tires begin rotating, they push against the Earth, and the Earth pushes back. As the vehicle gains angular momentum in one direction, the Earth gains angular momentum in the opposite direction. (RODGER BOSCH/AFP via Getty Images)

It’s not something you think of very often, but this plays out all the time in our reality. When you accelerate your automobile from a standstill as soon as the light turns green, your tires start to spin and push against the road. The road, therefore, exerts a force on the bottom of your tires, which causes your spinning tires to grip the road, accelerate, and push the car forward. Because the force isn’t directly on the center of the wheels — where the axels are — but rather off-center, your tires spin, gripping the road, and creating a torque.

But there’s an equal-and-opposite reaction here, too. The road and the tires have to push on one another with equal and opposite forces. If the force of the road on the tires causes your automobile to accelerate and then move, say, clockwise with respect to the center of planet Earth, then the force of the tires on the road will cause planet Earth to accelerate and rotate, ever so slightly, a little bit extra in the counterclockwise direction with respect to how it was moving before. Even though:

  • the car now has more angular momentum than it did before,
  • and the Earth now has more angular momentum than it did before,

the sum of the car+Earth system has the same amount of angular momentum as it did initially. Angular momentum, like force, is a vector: with magnitude and direction.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, . [+] represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter normal matter plays only a minor role. Once the structure collapses, however, the complex physics of normal matter becomes vitally important.

Ralf Kähler and Tom Abel (KIPAC)/Oliver Hahn

So what happens, then, when the large-scale structure in the Universe forms?

As long as you’re not too large for gravitational collapse to occur — where matter in the Universe can contract all the way down in one or more dimension to a scale where things will go “splat” due to collisions — these tidal torques will cause clumps of matter to pull on one another, inducing a rotation. This means that planets, stars, solar systems, galaxies, and even, in theory, entire cosmic filaments from the cosmic web should, at least sometimes, experience rotational motions. On larger scales, however, there should be no overall rotation, as there are no larger bound structures in the Universe.

This is precisely what the latest study sought to measure, and precisely what they found. For individual filaments, they couldn't see anything, but when they took thousands of filaments together, the rotational effects clearly showed up.

“By stacking thousands of filaments together and examining the velocity of galaxies perpendicular to the filament’s axis (via their redshift and blueshift), we find that these objects too display vortical motion consistent with rotation, making them the largest objects known to have angular momentum. The strength of the rotation signal is directly dependent on the viewing angle and the dynamical state of the filament. Filament rotation is more clearly detected when viewed edge-on.”

While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, . [+] the feedback from normal matter (red) can severely impact galactic scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Fascinatingly enough, the filaments that trace the lines connecting galaxy clusters appear to be rotating themselves.

Illustris Collaboration / Illustris Simulation

We’ve seen “filament rotation” before: in the filaments that are created in star-forming regions within individual galaxies. But in a surprise to some, even the largest-scale filaments in the Universe, the ones that trace the cosmic web, appear to be rotating as well, at least on average. Their speeds are comparable to the speeds at which galaxies move and stars orbit within the Milky Way: up to

hundreds of kilometers per second. Even though there’s a lot we still have left to unpack about this phenomenon, these large-scale cosmic filaments, which typically extend for hundreds of millions of light-years, are now the largest known rotating structures in the Universe.

Why are they rotating, however? Is it something that can truly be explained by tidal torques and nothing else? The early evidence points to “yes,” as the presence of large masses near the filaments — what cosmologists identify as “haloes” — seems to intensify the rotation. As the authors note, “the more massive the haloes that sit at either end of the filaments, the more rotation is detected,” consistent with gravitational torques inducing these motions. Nevertheless, more study is needed, as temperature and other physics may also play a role.

The big breakthrough is that we’ve finally detected rotation on these unprecedentedly large scales. If all goes well, we’ll not only figure out why, but we’ll be able to predict how quickly each filament that we see ought to be spinning, and for what reason. Until we can predict how every structure in the Universe forms, behaves, and evolves, theoretical astrophysicists will never run out of work to do.

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