Are the “missing baryons” simply too hot to see? | Hacker News Reader

Are the “missing baryons” simply too hot to see?

bigthink.com

51 pointsraattgift30 comments

Where is the actual article though? It's basically a million captioned images without any kind of article between them...?

"Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.", from the end of the article.

Maybe you'd want to consume different content if that's not for you?

Edit: beside that, they are cheating a bit with all these detailed image captions ;)

I probably do, hence checking "hacker news", not "hacker visual novels".

Thank you! My comment below got flagged for saying this. I can't understand why the fuck anybody upvoted this.

Neither their nor your or my taste for content is universal? They might like this form and instead hate Youtube Shorts, long form articles or scientific papers instead.

The theory is that only 1 billionth of the original matter/antimatter mixture survived as matter.

The rest annihilated. But when something annihilates the energy doesn't vanish - so where did that energy go??? One person I asked told me it bacame the CMB, but that seems far too weak for the amounts we are talking about.

AIUI, what you're missing is that it's an iterative process. Energy comes together and forms a hair more matter than antimatter, and the rest returns to energy. But this wasn't an "exactly one chance" situation; the energy gets a fresh chance to come together and a bit more of it is matter than antimatter, with the rest going back to energy... and so on and so on.

If it was perfectly balanced, no amount of iteration would produce matter, but even slight imbalances end up tipping the scale greatly over a lot of iterations.

(And of course it was all just a big mess of things, not a clean series of events like my words imply. But that's the idea.)

This isn't really a mystery. Mostly annihilating matter/antimatter becomes neutrinos or photons. The neutrinos became the cosmic neutrino background, which we have no means of detecting. The CMB wasn't weak when it first decoupled from matter. You're looking now at something that has been red-shifted by 13 billion years of the expansion of space until it's in microwave wavelengths now. When it was first created, it was energetic enough to ionize all matter in the universe for nearly 400,000 years before it finally decoupled.

It must have been wild, at some point in time, to look up from an atmosphere-free body and see that the background color of outer space was blue.

If you stretch out the CMB across the volume of the entire universe, it seems plausible it would be pretty faint. That r^-3 term on the energy density is pretty small when r is of order ~100 billion ly or more.

And there's another factor of r from the redshift of those photons.

Photons are redshifted as the universe expands, so the total energy in CMBR photons declines with time.

My personal pet theory is that it's in the kinetic energy of relative motion of galaxies.

The standard way of looking at things is that galaxies are roughly locally stationary and spacetime between them just inflates. But you could take the opposite look, which someone told me has exactly the same mathematical representation, that galaxies are moving away due to their kinetic energy and sort of drag along the spacetime between them, stretching it.

Also I'm not sure about matter-antimatter asymetry. We might be just in a place that locally had a little more matter than antimatter initally. This place might even be the size of the observable universe but we don't know how small part of entire universe is the observable universe. The thesis about matter and matter asymetry comes from the fact that we kinda assume that initially universe was pure energy. But we really can't tell. And everything that doesn't check out in cosmology might be the result of initial conditions being slightly different than the simplest ones we assume.

> that galaxies are moving away

moving away from what? The reason inflation works is that it doesn't require a privileged frame of reference.

From each other.

Bigbang that has a center is perfectly consistent with Hubble law. Imagine that the spacetime is flat, there's no gravity and clumps of matter (galaxies) initially had random velocities. The faster the galaxy was, the further is went till now. What is a bit surprising, is that if you do the math in such universe and compare distances from any galaxy to others and their relative velocities you'll get Hubble law exactly. For every galaxy the distance to any other galaxy is directly proportional to relative velocity between them and they appear to be moving directly away from each other. So even though this big bang had epicenter you can't really tell that it had it or where it was if you can't see the edge. And if our observable universe is small when compared with the size of the entire universe we have a low chance of being near the edge.

Try it. You can do it on paper or do a little computer simulation and measure. I did it few years ago. I was quite surprised with the results. Especially with the direction of the relative velocity vectors. They always face directly away from one galaxy to any other.

> From each other. Bigbang that has a center is perfectly consistent with Hubble law.

So we see all distant galaxies moving radially away us. Are you saying that the big bang happened to be centered, of all the universe, around the Milky Way?

No. I'm saying that if big bang imparted random velocities on galaxies that were all initially in the same spot (epicenter) then tofay in the reference frame of any galaxy (also Milky Way) it will look like every other galaxy is moving away from it radially with the velocity proportional to the current distance from it. No one will be able to tell where the actual epicenter was if they are too far from the edge to see it.

This holds for the universe without gravity, with flat spacetime, Newtonian physics, with three, two or one spatial dimension.

Seriously, try to simulate such toy universe. Or just sit down with a piece of paper and consider variant with one spatial dimension. It's primary school math and physics. You'll get Hubble law.

You’re right about the distribution of the velocities of galaxies. The problem would be the distribution of the density of galaxies. Even in an instantaneous snapshot of the state of the universe now, there would be more galaxies in the direction of the epicentre from us, say assuming we had average velocity. To see an even distribution of actual galaxies (not their velocities) in the sky we’d need to have zero initial velocity.

But also light would still take time to travel. Our view of galaxies 13bn light years away would still be of them as they were 13bn years ago in the very early universe. Again unless we happened to have zero initial velocity, we’d see all the distant galaxies clustered together in one spot in the sky.

We don't know of the distribution of galaxies in our universe is exactly equal. And the exact amount of any anisotropy is highly dependant on how small fraction of the whole universe is observable by us and how far are we from epicenter and the edge. 13bn light-years worth of galaxies might be too small sample to notice any asymetry with our current equipment. What if that's just one billionth of all the matter thrown apart by the big bang? Universe did not necessarily start from a point, just from a very energetic state where the all matter we can see now was fairly close together.

We can see the relative motion of galaxies up to the edge of the observable universe from their red shift. So we know they were all 'in one place' 13 bn years ago. It also means we know what the distribution of initial vectors must have been in this scenario for these galaxies.

So either the conventional view is correct and it's space that has expanded moving the galaxies further apart. This is consistent with a potentially infinite universe. Alternatively all the galaxies we can see were clustered together in a tight lump 13bn years ago and flew apart.

If the latter is the case the origin point would still be visible to us. As long as our initial random velocity was below the speed of light, we would always still be able to see light coming to us from the area of the origin point. With a random initial velocity distribution the fastest half of galaxies would be spread over a volume 4x bigger than that of the slowest half of galaxies. That means galaxy density in th direction of the origin point would be something like 4x higher that it is in directions away from the origin point. But as far as we can tell galaxy and galaxy cluster distribution is even.

Then there's the cosmic microwave background. If this is the after glow of the big bang, we wouldn't see it. All the radiation would have spread out in a shell at the speed of light. So we'd need tome other explanation for the CMB and why it happens to have all the exact characteristics we predicted for a big bang after glow.

Where does that 4x come from? This should be noticeable on the simple computer simulation that I made. It wasn't. When I "focused" the camera on a galaxy that had average speed it looked exactly as if it was the epicenter of the whole system if I limted my range of view to end right before the edge. With 4x assymetry in density I should be able to easily tell where the true epicenter was. I couldn't. I'm sure I could use some probability distribution that would show that effect, also gravity could make it more pronounced but uniform distribution without gravity was perfectly symmetrical.

My other argument is that if big bang didn't start in a single point then the real centerpoint of it might have never been inside the bubble of our observable universe due to speed of light and space-time expansions as the galaxies moved away.

CMB is just the image of hot matter that was close to our location 13bn years ago. Same as faint, highly redshifted images of galaxies from nearly that time. Redshift is mostly from the speed of that matter relative to us back then and spacetime expanding as it was "dragged" by the matter since then. CMB is unique in a sense that it is the image of the last time when all matter around us was so hot and dense that it was opaque. But it doesn't need any special mechanism and it's radiation is no different from our point of view than light from old galaxies.

The article was interesting but didn't explain why hot atoms would be invisible.

I think pop-sci articles should be (peer) reviewed by lay-people so questions like these can be addressed before the article is published.

This is the most exciting science event to follow recently.

Just wait until the normal media picks up on it. A story made for TV.

Are you sure you didn't mean to write that in one of the Superconductor articles?

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