We can measure dark energy across the universe in our own cosmic backyard

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The Andromeda galaxy is on a collision course with the Milky Way, and scientists are using that to study dark energy.

An illustration of deep space, filled with stars and dense orange/brown clouds.
(Image credit: agsandrew/Getty Images)

Researchers have found a new way to measure dark energy — the mysterious force that is causing the expansion of the entire universe to accelerate — using data from our own cosmic backyard.

Ever since its discovery in the late 1990s, dark energy has become the premier problem in cosmology. In short, we have no idea what dark energy is. But whatever it is, it's directly responsible for the accelerated expansion of the universe, and cosmologists have devised a wide variety of tools to try to learn more about it. 

For example, astronomers can study the brightness of distant supernova explosions to measure how quickly they are moving away from us. They can look back into the early days of the cosmos and determine the fundamental ingredients playing major roles back then. They can even map out the evolution of the largest structures in the universe, teasing out the effects of dark energy on that evolution.

Related: We have never seen dark matter and dark energy. Why do we think they exist?

The main challenge with all those techniques is that they require both deep and broad measurements, probing vast volumes of the universe. That makes sense; we don't feel the effects of dark energy in the solar system or even in the Milky Way galaxy, because overall, dark energy is a very weak effect and is easily swamped by the strong forces operating inside galaxies.

But now, a trio of researchers has found a way to use surprisingly small scales to measure dark energy. David Benisty, Anne-Christine Davis and N. Wyn Evans from the University of Cambridge discuss the technique in a paper accepted for publication in The Astrophysical Journal Letters and available as a preprint on arXiv. Benisty also discusses the technique in a seminar available on YouTube.

The technique is based on the fact that dark energy affects the relationships between every pair of galaxies. Galaxies naturally want to attract each other, pulling each other close with their strong gravity. But counteracting that natural impulse to clump together is dark energy itself, which acts like an antigravitational force that drives galaxies away from each other.

If galaxies are already close enough, then with enough time, they will overwhelm dark energy and crash together. But if they're too far apart, then their mutual gravitational attraction will never be enough to counteract dark energy, and they will be forever ripped apart.

Two galaxies colliding in space. (Image credit: NASA, Holland Ford (JHU), the ACS Science Team and ESA)

The nearest galaxy to the Milky Way is the Andromeda galaxy, which sits about 2.5 million light-years away. The two galaxies are on a collision course and will eventually begin merging in about 5 billion years. But this collision won't be directly head-on. The two galaxies slowly orbit each other as they draw closer to each other, taking about 20 billion years to complete a full circuit — which means we won't even complete a single full orbit before the collision and merger begin.

The mutual gravitational attraction is far too strong for dark energy to stop that, but the researchers discovered that the presence of dark energy in the cosmos affects the orbit of the two galaxies around each other and the eventual impact time. So we can use measurements of the precise position and motion of Andromeda to get a handle on dark energy, without having to go out into the wider universe.

Related stories:

— 1,500 supernovas offer most precise survey of dark energy and dark matter to date

— Mysterious dark energy is spread evenly across the cosmos

— Black holes may be the source of mysterious dark energy

The technique is still in its infancy, however. To use Andromeda to measure dark energy, we must have excellent measurements of the mass of both the Milky Way and Andromeda. The more uncertain we are in those measurements, the less precise we can be about the impact of dark energy on our mutual orbit.

So, although the astronomers weren't able to deliver more precise measurements of dark energy, they hope that future refinements of the technique, plus applications to more pairs of colliding galaxies, will help us solve this perplexing dark energy mystery.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Paul Sutter
Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

12 Comments Comment from the forums
  • rod
    Some good comments about DE near the end of the article.

    "The technique is still in its infancy, however. To use Andromeda to measure dark energy, we must have excellent measurements of the mass of both the Milky Way and Andromeda. The more uncertain we are in those measurements, the less precise we can be about the impact of dark energy on our mutual orbit. So, although the astronomers weren't able to deliver more precise measurements of dark energy, they hope that future refinements of the technique, plus applications to more pairs of colliding galaxies, will help us solve this perplexing dark energy mystery."

    The arxiv.org link provided shows the letter is working with the cosmological constant used in GR math for expanding space.

    Constraining Dark Energy from Local Group dynamics, https://arxiv.org/abs/2306.14963
    "This Letter develops a method to constrain the Cosmological Constant Λ from binary galaxies, focusing on the Milky Way and Andromeda."

    As I understand the cosmological constant in GR, there is at least a 10^120 magnitude error problem between GR and quantum mechanics for vacuum energy in the Universe. Just a bit more cosmological constant at the beginning in the BB model, space expands so fast there is no Universe today :) Some other interesting points to remember here too.

    5 fascinating facts about the Big Bang, the theory that defines the history of the universe, https://www.livescience.com/space/cosmology/5-fascinating-facts-about-the-big-bang-the-theory-that-defines-the-history-of-the-universe
    My observation. Interesting 5 point *fascinating facts about the Big Bang* presented in this livescience.com report. I note on point #5, "...Similarly, the Big Bang wasn't an explosion in space — it was an explosion of space. The Big Bang happened to everything in the universe simultaneously. It did not happen in one particular location in space, but in a particular location in time. It's hard to think about, but that's why we have mathematics: to help us grapple with concepts we normally couldn't."

    My thought, this is an instantaneous-action-at-a-distance force used in the BB model to explain the origin of space, *everywhere*, thus no center to the expanding universe model. Also, the conservation law of energy is violated, energy appears at a moment in time. Today a GRB documented released about 10^55 erg, how much energy was released at the moment of the Big Bang? Now we have the instantaneous-action-at-a-distance force too for expanding space to appear, *everywhere*. We also have the fine tuning needed for the cosmological constant used in expanding space math too.
    Reply
  • billslugg
    As I understand it, there has never been a conservation violation, right on back to the instant of creation. The negative gravitational energy exactly balanced the mass/energy created. I don't really understand it but I read that once somewhere.
    Reply
  • rod
    Yes, negative energy apparently is used to get around violating the law of conservation of energy in the BB model.

    https://forums.space.com/threads/total-energy-of-the-system.61662/
    Some comments in this discussion on the forums about it. Much effort by Lawrence Krauss to get around this problem in BB model, just like instantaneous-action-at-a-distance to explain space appearing after BB moment, everywhere. When it comes to negative energy, I think this is inflation that uses the inflaton :)
    Reply
  • rod
    billslugg said:
    As I understand it, there has never been a conservation violation, right on back to the instant of creation. The negative gravitational energy exactly balanced the mass/energy created. I don't really understand it but I read that once somewhere.
    Alan Guth teaches this in inflation.

    WAS COSMIC INFLATION THE 'BANG' OF THE BIG BANG?, https://ned.ipac.caltech.edu/level5/Guth/Guth_contents.html
    https://ned.ipac.caltech.edu/level5/Guth/Guth3.html, "3. THE INFLATIONARY UNIVERSE...Once a patch of the early Universe is in the false vacuum state, the repulsive gravitational effect drives the patch into an inflationary period of exponential expansion. To produce a universe with the special features of the Big Bang discussed above, the expansion factor must be at least about 10^25. There is no upper limit to the amount of expansion. Eventually the false vacuum decays, and the energy that had been locked in it is released. This energy produces a hot, uniform, soup of particles, which is exactly the assumed starting point of the traditional Big Bang theory. At this point the inflationary theory joins onto the older theory, maintaining all the successes for which the Big Bang theory is believed. In the inflationary theory the Universe begins incredibly small, perhaps as small as 10^-24 cm, a hundred billion times smaller than a proton. The expansion takes place while the false vacuum maintains a nearly constant energy density, which means that the total energy increases by the cube of the linear expansion factor, or at least a factor of 10^75. Although this sounds like a blatant violation of energy conservation, it is in fact consistent with physics as we know it. The resolution to the energy paradox lies in the subtle behavior of gravity. Although it has not been widely appreciated, Newtonian physics unambiguously implies that the energy of a gravitational field is always negative a fact which holds also in general relativity. The Newtonian argument closely parallels the derivation of the energy density of an electrostatic field, except that the answer has the opposite sign because the force law has the opposite sign: two positive masses attract, while two positive charges repel. The possibility that the negative energy of gravity could balance the positive energy for the matter of the Universe was suggested as early as 1932 by Richard Tolman, although a viable mechanism for the energy transfer was not known. During inflation, while the energy of matter increases by a factor of 10^75 or more, the energy of the gravitational field becomes more and more negative to compensate. The total energy - matter plus gravitational - remains constant and very small, and could even be exactly zero. Conservation of energy places no limit on how much the Universe can inflate, as there is no limit to the amount of negative energy that can be stored in the gravitational field. This borrowing of energy from the gravitational field gives the inflationary paradigm an entirely different perspective from the classical Big Bang theory, in which all the particles in the Universe (or at least their precursors) were assumed to be in place from the start. Inflation provides a mechanism by which the entire Universe can develop from just a few ounces of primordial matter. Inflation is radically at odds with the old dictum of Democritus and Lucretius, "Nothing can be created from nothing" If inflation is right, everything can be created from nothing, or at least from very little. If inflation is right, the Universe can properly be called the ultimate free lunch."

    Some very fine tuning is needed in all of this *physics* including a Universe 10^-24 cm size in the beginning :)
    Reply
  • Torbjorn Larsson
    It remains odd when articles claims that "we have no idea what dark energy is", while their references such as the video goes on to state that we have a great deal of ideas. And not only that but with such observations as the equality between the speed of gravity waves and the speed of light cosmologists has narrowed down the field immensely. What the author meant to say was likely "we have not decided what dark energy is", but that is a different problem.

    @rod: There is no "error" in the cosmological constant. (Or between general relativity or quantum field theory since gravity can be quantized as other forces: "Quantum gravity as a low energy effective field theory", Donoghue, Scholarpedia.) The vacuum energy density was initially thought to be zero, but dark energy showed it wasn't. If we also introduce the quantum fields vacuum expectation values there is a problem with Planck scale terms near canceling, which is unlikely ("unnatural") but possible. Weinberg's anthropic multiverse, which naturally follows from the currently observed scalar quantum inflation field , solved that problem in the 80s and predicted the later observed value of dark energy (but few wanted to accept such solutions).

    There is also no "explosion" in the 10^-5 parts - as seen in the cosmic background radiation - homogeneous and isotropic universe. Inflation expansion was rapid, but smooth.

    Nor is there "an instantaneous-action-at-a-distance force" or "a conservation law of energy ... violated" in current cosmology, it is all relativistic non-Newtonian physics and general relativity has no general energy measure. There is practically no center - it is not necessary to understand LCDM cosmology and not something we can observe - but if you scale out to the bubble universe we live in its spatial volume has a center in theory. General relativity Einstein Field Equations show that it has an energy conserving Lagrangian in every point in space, but its tensor solutions has no general energy measure. (So they are approximations in that sense, but the quantum theory is the more fundamental one while general relativity provides easier solutions.) That is why the video in the beginning discuss how gravity theories, like quantum field theories. need to check for "ghosts" - solutions that have negative particle energies.

    @ rod, billslugg: While general relativity is described as "has no conservation of energy" due to the problems of defining solution energies - see above - there are situations where you can apply general field theory perfectly or as an approximation. Remember that both general relativity and quantum field theories are known to be "effective" theories that apply at low energies, at Planck scales quantum field theory admits no low energy perturbation particle solutions and general relativity no locally flat solutions for quantum field theory to work in. But if you stay away from such problems - and inflation makes that possible - you have always a total energy measure in general relativity https://en.wikipedia.org/wiki/Friedmann–Lemaître–Robertson–Walker_metric ] and outside of masses a stationary vacuum solution has a gravitational field potential. The gravitational field potential energy is negative, as rod's reference discuss, and in vacuum it balances the other field's positive potential energies through the Einstein Field Equations - no need for Guth's energy transfer. While Guth at the time ingeniously was showing how inflation would solve a lot of problems I think modern observations of its nature as well as the flat space nature of the universe points to a more "natural" solution of quantum field nature.

    The size of the universe can be debated. For the observable universe it used to be said "football sized" after inflation but newer observations gives me "room sized" when I do the estimate. We can also estimate the local hot big bang universe original size. First we can take the largest cosmological filament of gas and galaxy clusters as a measure of the last inflation fluctuations at it reached the end of the slow roll and went into the hot big bang era. Then we can apply BICEP/Keck data on its potential energy so we can estimate the expansion during the slow roll. We then get that our bubble universe arose from an inflation quantum fluctuation that pushed it into slow roll that is now 10^75 times larger in volume than the observable universe.* But of course the same data imply that the inflationary multiverse is infinite in volume, unless you finetune it so that can't happen (which is difficult and "unnatural").

    EDIT: Removed erroneous remark on expansion.
    Reply
  • rod
    Torbjorn Larsson said:
    It remains odd when articles claims that "we have no idea what dark energy is", while their references such as the video goes on to state that we have a great deal of ideas. And not only that but with such observations as the equality between the speed of gravity waves and the speed of light cosmologists has narrowed down the field immensely. What the author meant to say was likely "we have not decided what dark energy is", but that is a different problem.

    @rod: There is no "error" in the cosmological constant. (Or between general relativity or quantum field theory since gravity can be quantized as other forces: "Quantum gravity as a low energy effective field theory", Donoghue, Scholarpedia.) The vacuum energy density was initially thought to be zero, but dark energy showed it wasn't. If we also introduce the quantum fields vacuum expectation values there is a problem with Planck scale terms near canceling, which is unlikely ("unnatural") but possible. Weinberg's anthropic multiverse, which naturally follows from the currently observed scalar quantum inflation field , solved that problem in the 80s and predicted the later observed value of dark energy (but few wanted to accept such solutions).

    There is also no "explosion" in the 10^-5 parts - as seen in the cosmic background radiation - homogeneous and isotropic universe. Inflation expansion was rapid, but smooth.

    Nor is there "an instantaneous-action-at-a-distance force" or "a conservation law of energy ... violated" in current cosmology, it is all relativistic non-Newtonian physics and general relativity has no general energy measure. There is practically no center - it is not necessary to understand LCDM cosmology and not something we can observe - but if you scale out to the bubble universe we live in its spatial volume has a center in theory. General relativity Einstein Field Equations show that it has an energy conserving Lagrangian in every point in space, but its tensor solutions has no general energy measure. (So they are approximations in that sense, but the quantum theory is the more fundamental one while general relativity provides easier solutions.) That is why the video in the beginning discuss how gravity theories, like quantum field theories. need to check for "ghosts" - solutions that have negative particle energies.

    @ rod, billslugg: While general relativity is described as "has no conservation of energy" due to the problems of defining solution energies - see above - there are situations where you can apply general field theory perfectly or as an approximation. Remember that both general relativity and quantum field theories are known to be "effective" theories that apply at low energies, at Planck scales quantum field theory admits no low energy perturbation particle solutions and general relativity no locally flat solutions for quantum field theory to work in. But if you stay away from such problems - and inflation makes that possible - you have always a total energy measure in general relativity https://en.wikipedia.org/wiki/Friedmann–Lemaître–Robertson–Walker_metric ] and outside of masses a stationary vacuum solution has a gravitational field potential. The gravitational field potential energy is negative, as rod's reference discuss, and in vacuum it balances the other field's positive potential energies through the Einstein Field Equations - no need for Guth's energy transfer. While Guth at the time ingeniously was showing how inflation would solve a lot of problems I think modern observations of its nature as well as the flat space nature of the universe points to a more "natural" solution of quantum field nature.

    The size of the universe can be debated. For the observable universe it used to be said "football sized" after inflation but newer observations gives me "room sized" when I do the estimate. We can also estimate the local hot big bang universe original size. First we can take the largest cosmological filament of gas and galaxy clusters as a measure of the last inflation fluctuations at it reached the end of the slow roll and went into the hot big bang era. Then we can apply BICEP/Keck data on its potential energy so we can estimate the expansion during the slow roll. We then get that our bubble universe arose from an inflation quantum fluctuation that pushed it into slow roll that is now 10^75 times larger in volume than the observable universe.* But of course the same data imply that the inflationary multiverse is infinite in volume, unless you finetune it so that can't happen (which is difficult and "unnatural").

    *I've now had time to read that part of Guth's article in New Wrights cosmology compendium. I happened to note - and it's in the quote here as well though I missed that at the time - that we get to the same estimate of 10^75 volume (or energy) expansion! I pulled the number of e-folds expansion from Planck data and checked that it is consistent with BICEP/Keck newer data. (Which covers the energy up to where the standard particle Higgs sector lose stability according to the top mass measured by LHC.) Guth likely knew the order of things from early theory as well.
    Torbjorn Larsson, you stated about the cosmological constant. "@rod: There is no "error" in the cosmological constant. (Or between general relativity or quantum field theory since gravity can be quantized as other forces: "Quantum gravity as a low energy effective field theory", Donoghue, Scholarpedia.) The vacuum energy density was initially thought to be zero, but dark energy showed it wasn't. If we also introduce the quantum fields vacuum expectation values there is a problem with Planck scale terms near canceling, which is unlikely ("unnatural") but possible. Weinberg's anthropic multiverse, which naturally follows from the currently observed scalar quantum inflation field , solved that problem in the 80s and predicted the later observed value of dark energy (but few wanted to accept such solutions)."

    As far as my readings, the cosmological constant remains a problem between GR and QM.

    Was Einstein wrong? Why some astrophysicists are questioning the theory of space-time, https://forums.space.com/threads/was-einstein-wrong-why-some-astrophysicists-are-questioning-the-theory-of-space-time.38956/
    Here is my post #2. "I found 19 references to *quantum* in this article, 0 references to *cosmological constant*. The Cosmological Constant Is Physics’ Most Embarrassing Problem, https://www.scientificamerican.com/article/the-cosmological-constant-is-physics-most-embarrassing-problem/, Feb-2021. Einstein GR and QM are in serious conflict here describing expanding space. Without the metrics from GR, there is no math describing the expansion of space, redshifts apparently, and cosmological distances using redshifts. Another comment in this report from Scientific American, “One of the first people to notice something was amiss was physicist Wolfgang Pauli, who found in the 1920s that this energy should be so strong that the cosmos should have expanded long past the point where light could traverse the distance between any of the objects in it. The whole of the observable universe, Pauli calculated, “would not even reach to the moon.” He was reportedly amused by his estimation, and no one took it seriously at the time. The first to formally calculate the value of the cosmological constant based on quantum theory's predictions for the vacuum energy was physicist Yakov Zel'dovich, who found in 1967 that the energy should make the cosmological constant gigantic.” That suggest using QM, space is expanding so fast, we should not be here today :)"

    As I understand inflation, nature must have the inflaton too, something not seen in particle experiments or anywhere in astronomy operating in nature today. There are other exotic particles in the new physics used to explain the origin of the Universe too like magnetic monopoles.
    Reply
  • rod
    Torbjorn Larsson in post #6 said about the size of the Universe, "The size of the universe can be debated. For the observable universe it used to be said "football sized" after inflation but newer observations gives me "room sized" when I do the estimate. We can also estimate the local hot big bang universe original size. First we can take the largest cosmological filament of gas and galaxy clusters as a measure of the last inflation fluctuations at it reached the end of the slow roll and went into the hot big bang era. Then we can apply BICEP/Keck data on its potential energy so we can estimate the expansion during the slow roll. We then get that our bubble universe arose from an inflation quantum fluctuation that pushed it into slow roll that is now 10^75 times larger in volume than the observable universe.* But of course the same data imply that the inflationary multiverse is infinite in volume, unless you finetune it so that can't happen (which is difficult and "unnatural")."

    I will stay the course with Alan Guth 1997 report and 2013 report I already cited. Alan Guth showed inflation began when the Universe we see today was 10^-51 cm size in 2013 (scale size 10^-53 m maps to 1 m size today) and earlier in 1997, 10^-24 cm size. The comoving radial distance for the CMBR redshift today using z=1100 and H0 = 69 km/s/Mpc is about 46 billion light years from Earth today.

    Edit note. 46 billion light years ~ 4.3519360E+28 cm or about 4.352 x 10^28 cm compared to the 1997 value for starting size of 10^-24 cm or 2013 value of 10^-51 cm size presented by Alan Guth. I prefer to see actually measurement sizes in cm for cosmology vs. a room size object, grapefruit size object or whatever. It is then easier for me to compare with present distances documented today in astronomy and the cosmology calculators.
    Reply
  • rod
    WAS COSMIC INFLATION THE 'BANG' OF THE BIG BANG?, https://ned.ipac.caltech.edu/level5/Guth/Guth_contents.html, the 10^-24 cm size for inflation stated here.

    How was the universe created? https://forums.space.com/threads/how-was-the-universe-created.59247/
    My post #6. "What was the size of the universe when inflation began? “A typical GUT-scale inflationary model would include about 60 e-folds of inflation, expanding by a factor of e^60 ≈ 10^26. From the end of inflation to today the universe would expand by another factor of ∼ 10^15 GeV/3K ≈ 10^27. This means that a distance scale of 1 m today corresponds to a length of only about 10^−53 m at the start of inflation, 18 orders of magnitude smaller than the Planck length (∼ 10^−35 m).” ref -https://ui.adsabs.harvard.edu/abs/2013arXiv1312.7340G/abstract, Inflation starts in a universe 10^-53 m size, 18 order of magnitudes smaller than the Planck length."
    Reply
  • rod
    The article about measuring DE in our backyard states - "The nearest galaxy to the Milky Way is the Andromeda galaxy, which sits about 2.5 million light-years away. The two galaxies are on a collision course and will eventually begin merging in about 5 billion years. But this collision won't be directly head-on. The two galaxies slowly orbit each other as they draw closer to each other, taking about 20 billion years to complete a full circuit — which means we won't even complete a single full orbit before the collision and merger begin. The mutual gravitational attraction is far too strong for dark energy to stop that, but the researchers discovered that the presence of dark energy in the cosmos affects the orbit of the two galaxies around each other and the eventual impact time. So we can use measurements of the precise position and motion of Andromeda to get a handle on dark energy, without having to go out into the wider universe."

    I enjoy reading reports like this and comparing the measurements to the beginning of the Universe in the BB cosmology, that includes the inflation paradigm too. Consider the distance and orbit period described above and timescale for the collision, all of this in astronomy originated in a Universe perhaps 10^-51 cm size or perhaps 10^-24 cm size in cosmology today, that is some model of origins.
    Reply
  • Classical Motion
    To me, dark matter and dark energy.....are like e0 and u0. e0 and u0 are the properties of space that limit light to c. But c is limited because of acceleration, not impedance. Space has no e0 or u0.

    We invented e0 and u0. If c had a different V, then e0 and u0 would change to satisfy it. Just like DM and DE. To fit a false narrative.
    Reply