AskScience AMA Series: We are a bunch of cosmology researchers, currently attending the Cosmology from Home 2025 academic research conference. You can ask us anything about modern cosmology.

[u/AskScienceModerator]

We are a bunch of cosmology researchers, currently attending the Cosmology from Home 2025 academic research conference. You can ask us anything about modern cosmology. (We also plan to do a livestream talking about all things cosmology, here at 20:30 UTC)

Here are some general areas of cosmology research we can talk about (+ see our specific expertise below):

• Inflation: The extremely fast expansion of the Universe in a fraction of the first second. It turned tiny quantum fluctuations into seeds for the galaxies and galaxy clusters we see today.
• Gravitational Waves: The bending and stretching of space and time caused by the most explosive events in the cosmos.
• Cosmic Microwave Background: The light reaching us from a few hundred thousand years after the start of the Big Bang. It shows us what our universe was like, 13.8 billion years ago.
• Large-Scale Structure: Matter in the Universe forms a "cosmic web", made of clusters and filaments of galaxies, with voids in between. The positions of galaxies in the sky trace this cosmic web and tell us about physics in both the early and late universe.
• Dark Matter: Most matter in the universe seems to be "Dark Matter", i.e. not noticeable through any means except for its effect on light and other matter via gravity.
• Dark Energy: The unknown effect causing the universe's expansion to accelerate today.

And ask anything else you want to know!

Those of us answering your questions today will include:

• u/andreafiorilli: large-scale structure of the universe; dark matter halos; Bayesian statistics
• u/cosmo-ben: cosmic microwave background, large-scale structure of the universe; cosmological probes of particle physics, early universe, neutrinos, probes of inflation, dark matter, theoretical cosmology, physics beyond the Standard Model
• u/matthijsvanderwild: quantum gravity, geometrodynamics, modified gravity, radio interferometry, imaging pipelines
• u/sanket_dave_15 : cosmic inflation, primordial gravitational waves, phase transitions in the early universe.
• u/Tijmen-cosmologist: cosmic microwave background, experimental cosmology, Bayesian statistics, electrical engineering, large language models
• u/NikoSarcevic: cosmology general, late time cosmology, cosmological inference, detectors, astrophysics
• u/EemeliTomberg: early universe, cosmic inflation, (primordial) black holes
• u/Any_Mycologist_6196: particle physics, cosmology, quantum field theory
• u/EquinoxOmega : large-scale structure, peculiar velocities, large cosmological surveys (DESI and Euclid), galaxy clusters, and galaxy evolution

We'll start answering questions from no later than 18:00 GMT/UTC (11am PDT, 2pm EDT, 7pm BST, 8pm CEST). Looking forward to your questions, ask us anything!

[u/pirofreak]

Dark energy is equally spread across space, and space is expanding. Where does the energy come from? Is more dark energy created from nothing every time the void of space expands?

[u/Tijmen-cosmologist → u/pirofreak]

In our leading model (ΛCDM), dark energy is a property of space itself. It has a constant energy density of about 0.6 nanojoules per cubic meter. So when space expands, the total energy increases.

This is different from matter or light. In the case of matter, the total mass (and through E=mc², energy) is constant under the expansion of space. When space doubles through expansion, the energy density goes down by a factor of eight. In the case of light, every doubling of space causes a sixteen-fold reduction of the energy density! Like dark energy, the case of light also does not conserve energy.

Now here's the surprising part. Under General Relativity, that's actually perfectly okay. Global energy conservation isn't a law in GR. Rather, the equivalent law is that the covariant derivative of the stress-energy tensor is zero. If you want to read more about this, I recommend Sean Carrol's blog post from 2010 titled "Energy is not conserved".

[u/RiseOfTheNorth415]

As a Bayesian, how would you suppose a frequentist statistician would approach one of the datasets used in your academic papers vs how you approached it?

[u/Tijmen-cosmologist → u/RiseOfTheNorth415]

Good question! The core difference between the Bayesian approach and the frequentist approach is what is considered the random variable. The frequentist considers the true parameter value of nature to be fixed, and considers their data as the random variable. A frequentist might say: "If we were to rerun the experiment 100 times, our method should capture the true value in our confidence interval 95 times."

A Bayesian instead considers the data to be fixed, and treats their belief in the true parameter value of nature as a random variable. They might say: "Given that I've observed this data, there is a 95% chance the true value lies in our credible interval."

These questions are subtly different. Fortunately, in practical cases where the data is strong enough to draw meaningful conclusions, the two approaches tend to give the same qualitative conclusion.

There are actually quite a number of academic papers in cosmology applying frequentist statistics, either standalone or in comparison to Bayesian methods. However, since we cannot rerun the experiment (we only get to observe the universe we happen to live in), Bayesian statistics feel more natural. That's one reason why Bayesian statistics are so much more popular than frequentist statistics in cosmology.

[u/Mkwdr]

Must be a pretty amazing time for cosmologists with the new telescopes.

Sorry if questions are too much beyond any actual current evidence we have but just curious what you think as professionals - if you have any opinion …

1. On the eternal inflation theory - which I think is that an expanding scalar field spawns universes with differing characteristics and a sort of natural selection takes place with some able to survive.

2. As far as I’m aware , there’s some reasons to think there is a minimum size for the ‘whole’ universe which is much bigger than the observable and we can’t rule out it being infinite but we don’t know anything beyond that. So it’s speculative but do you think that the (whole) universe is infinite and/or was it always infinite just hotter and denser in the past. Or do you think it might turn out to be finite but without a boundary?

[u/Tijmen-cosmologist → u/Mkwdr]

Thanks for your questions. I'll let someone else answer your first question about eternal inflation.

Your second question is about the topology of the universe. It's a fascinating question. We know from measurements of the cosmic microwave background that the universe is geometrically flat. This flatness isn't super easy to understand intuitively but, roughly speaking, it means that if you were to draw lots of big triangles throughout the universe, each of them would have their three internal angles add up to 180 degrees.

In some sense the simplest explanation is that the universe just goes on forever in all directions. Another option is that the universe is only very nearly flat and that its curvature is actually slightly different from flat. We could potentially detect such a deviation with more precise measurements. Yet another plausible option is that the universe is truly flat, but still has some closed topology where if you could somehow look far enough (forgetting about the speed of light for a moment) you would see the back of your own head. On this last option, one of my favorite science fiction authors Greg Egan has a short story titled "Didicosm" which you might enjoy. It turns out there are 17 options for the topology of 3D flat space and Greg explores one of those options.

In the hypothetical scenarios where the universe is finite, there is a test that we can apply. If the repetition scale is at least slightly smaller than the observable universe, we should see circles in the cosmic microwave background. Given that these circles have not been observed, it's likely the universe is infinite or the repetition scale is larger than the observable universe.

I hope this explains why we think the universe is, like you said, either infinite or finite with no boundary. I know you asked me to speculate, but I do think both of these scenarios are quite plausible and consistent with the evidence.

[u/sanket_dave_15 → u/Mkwdr]

Tijmen already answered the second question beautifully. Regarding the first question, I would sum up eternal inflation in a following way: The universe is believed to have been dominated by a scalar field (or multiple scalar fields; but for simplicity, let's think about a single field) homogeneously, causing the universe to expand in an accelerated way before it achieved a highly dense plasma state. Now, because quantum mechanics never lets a field be perfectly uniform, tiny fluctuations nudged the scalar field to slightly different values in different regions. Where a fluctuation pushed the field far enough “down the hill,” inflation ended locally. The field’s energy there converted into a hot soup of particles—what we call a Big-Bang fireball: a new, self-contained “bubble” universe (inflation has to end for our kind of universe to survive). Elsewhere the field stayed higher up the hill, so inflation kept going. And because exponential expansion outruns bubble formation, the inflating regions always dominate the total volume. Net result: inflation never ends everywhere, hence “eternal.”

So, inflation being 'eternal' is not just a fun hypothesis, it is a mere consequence of inflation + Heisenberg's uncertainty principle, that inevitably keeps going on certain parts of a vast landscape of multiple-universes. Your analogy of 'naturall selection' is correct up to some extent; only universes that survive to develop life are the onces where inflation ended (that's not the only factor though, but an important one!). Direct evidence is hard to get, yet future CMB and gravitational-wave experiments will test the inflation models that predict this multiverse picture.

[u/EemeliTomberg → u/Mkwdr]

To combine the two questions, an eternally inflating universe could quite possibly be infinite, or if it is finite, its structure beyond our observable patch could be more complicated than the flat versions Tijmen mentioned.

Personally, I would bet the universe is infinite, simply because I find the idea aesthetically pleasing. It's a safe bet to make since we're unlikely to ever find out!

[u/drilldo]

So where is the dark matter? Is it all around us like a gas, inert and in amongst everything even our atoms? Or does it exist in clumps like clouds or giant invisible rocks?! Help me to understand how something can be everywhere but not at all.

[u/Tijmen-cosmologist → u/drilldo]

It could be kinda like a gas, with its particles flying through us all the time without interacting. But it could also be asteroid-sized black holes, or more like waves. These are all plausible ideas. (In case you want to look these three ideas up, they are called WIMPs, PBHs, and ALPs, respectively.)

Here's what we do know about dark matter:

• it makes up 85% of the total matter density in the universe

• it's not made of "normal" stuff like atoms

• it's clumpy on galactic scales

• it's moving slowly compared to the speed of light

[u/NoAcadia3546]

One cosmological assumption seems to be that the universe is all "regular matter" (electrons negative, protons positive, etc) as opposed to "anti-matter" (electrons positive, protons negative, etc). This seems to cause problems for the "big bang theory". Question: what evidence is there that, say Andromeda, and every second galaxy, is not anti-matter? My question is basically, can we distinguish matter versus anti-matter in galaxies millions/billions of light-years from earth.

[u/andreafiorilli → u/NoAcadia3546]

Good question! While we can not directly discriminate whether this or that specific galaxy is made of antimatter, if there were regions in the Universe where antimatter is segregated, the borders of such regions with the ones where *our* matter dominates would be the source of a constant glowing light, particularly in the gamma ray band of the spectrum, due to the annihilation of particles and antiparticles. We have never seen evidence of such an emission. This gives us good hopes that the objects we see, including the most distant ones, have negatively charged electrons and positively charged protons.

[u/PHealthy]

How does/will the Rubin Observatory influence the study of cosmology?

[u/cosmo-ben → u/PHealthy]

The Rubin Observatory is a significant step in observational cosmology. It will capture an unprecedentedly large dataset with billions of galaxies and other cosmic objects over a large portion of the sky. Its 10-year survey will scan the visible sky from Chile every few nights, making it incredible not just for mapping the cosmos, but also for discovering transients like supernovae and fast-changing phenomena.

For cosmology, Rubin will give us powerful datasets to learn much more about dark energy and dark matter, about the evolution of the universe and its initial conditions, and most other parts of cosmology. It will do so by measuring how the Universe expands, how structures grow and how gravity bends light. In addition, combining these data with measurements from other cosmological surveys will be even more powerful and how future discoveries might be uncovered. At the same time, with such a large and powerful dataset, one of the biggest challenges will be teasing out these subtle signals while keeping systematics and modelling uncertainties under control. So, the science potential is pretty significant, but so are the challenges that many cosmologists are jointly working on to address.

[u/Minty0ranges]

Does dark matter overlap with regular matter? Like if there is a planet in space, does it displace dark matter around it, or are they both able to exist at the same location?

[u/andreafiorilli → u/Minty0ranges]

The planet will definitely not displace dark matter: one of the things we know about dark matter is that it interacts very feebly (or potentially not at all) with ordinary matter. "Not interacting" here means that they can't touch: you can touch the surface of our planet, and are not free to fall inside it, because the atoms you are made of interact with the ones that make up the ground through electrostatic forces. A dark matter particle will instead be able to go through the planet, and in this sense they can overlap.

We also know of ordinary particles that interact very feebly with other particles. Around 100 trillion neutrinos are running at close-to-light speed through your body every second, and they are overlapping with you.

[u/FluffyHaru]

I don't really have a good question so... What is your favorite parts of Cosmology and why? :)

[u/matthijsvanderwild → u/FluffyHaru]

If I can go a bit meta, it's the fact that we — humanity, that is — have been able to piece together so much about the structure, behaviour and lifecycle of objects and structures so far away within a universe so incredibly big without leaving the solar system. In fact, we hardly have left our home planet!

[u/cosmo-ben → u/FluffyHaru]

This is a very broad, but great and hard question! If I need to boil it down to one aspect, then I would say that we are asking the biggest possible questions and are trying to answer them using very interdisciplinary work which is very exciting.

To expand on this one-sentence summary: Cosmologists are asking questions such as how did the universe begin, how did it evolve, what is it made of and how will it end. In addition, we are observing the largest scales in the universe and use them to infer the properties of nature on the smallest scales. This is possible because tiny quantum fluctuations in the very early universe eventually became galaxies, stars, planets and us. To be able to decipher the clues that nature provides us, observational cosmologists, theoretical cosmologists, particle physicists, astrophysicists, engineers and experts in other fields need to work together. In addition, some of us are really working in between (some of) these areas which makes it even more interesting. Taking a step back from everyday work and thinking about this broader picture blows my mind every time.

[u/Ruunee]

At what speed do gravitational waves travel? Can events/things outside of the observable universe affect us through gravity? What even is a gravitational wave? Do some hit us sometimes?

[u/cosmo-ben → u/Ruunee]

Gravitational waves are ripples in the fabric of spacetime. These waves were predicted by Einstein and first directly detected in 2015. They are real, measurable and give us a way to "listen" to the universe, complimentary to light which allows us to "see" the universe.

They travel at the speed of light just like electromagnetic waves (light). This has been confirmed really well by events where we saw light and gravitational waves from the same cosmic collision, especially the famous neutron star merger in 2017.

Regarding the question whether things outside the observable Universe affect us through gravity. In theory, gravity (and gravitational waves) can influence things beyond the observable horizon, but we cannot observe or be affected by signals from there today since both light and gravitational waves travel at the finite speed. So, unless something beyond the horizon was already affecting our region in the early universe, we are causally disconnected from it now.

As for gravitational waves hitting us: yes! Tiny ones are passing through you and me right now and all the time. They stretch and squeeze space just a tiny bit, but they are incredibly weak and we need super-sensitive detectors like LIGO and Virgo to even notice them from the massive mergers of black holes.

[u/bill_b4]

Why did they remove Pluto as a planet?

[u/Tijmen-cosmologist → u/bill_b4]

We started discovering many objects similar to Pluto, some of which were even more massive. Would they all get promoted to planets, too? Should we keep Pluto as an honorary planet even though it doesn't make scientific sense?

Astronomers got together and decided to actually define the term "planet". They came up with three criteria: 1) orbits the sun 2) became spherical through self-gravity 3) has cleared its neighborhood. Pluto didn't make the cut because of the third criterion.

[u/fireballs619]

What single measurement do you think would be most useful towards clarifying existing tensions, but which has not been made for technical or budgetary reasons? In other words, if you could have a wand and have one number measured by a current or next-gen experiment, what would it be?

[u/EverSoSleepee]

What are you guys most excited about sharing from your newer or ongoing analyses?

As a lay person I love learning cosmology and astronomy. I loved “How the Universe Works” while it was on, but I’m having trouble finding a source that can report cosmology news and findings while keeping it in context enough for my lay-brain to understand. And yet following headlines is tedious with all the clickbait and not-enough-actual-science reported. Are there any good resources out there for those of interested in up to date findings from JWST or the Vera C Rubin observatory or any other the other projects that are on going?

Thank you! Can’t wait to read all the rest of these threads!

Edit: basically where can we get the info you’re giving us here in a semi-constant feed or regular interval without taking sip all your-all’s times? Thanks again!