r/cosmology

During our weekly cosmology discussion salon a big brouhaha developed to the extent that people were shouting.

I proposed that most matter in the universe is invisible, exotic, and still undetected directly.

My friend, a retired physicist firmly believes that gravity itself is being misread at galactic/cosmic scales. He said my theory relies on too many patches to be truth and I’m being too subjective.

I’m curious what the crowd source consensus is : particle dark matter, or modified gravity/MOND-like theories?

When I try to imagine the size of the observable universe, I don’t feel uncomfortable. But when I think that the actual universe could be hundreds of times larger than the observable universe, or maybe even truly infinite, I genuinely start to feel dizzy and strange.. Is this some kind of anxiety or obsession, or does everyone feel this way? Are there any estimates about the true size of the universe? Or could it really be infinite in the literal sense that we understand, not just mathematically or logically, but truly endless with no edge or end? If so, how?

Hello everyone. I am a teenager and I just started being interested in space and cosmology. so I'm sorry if my theory is unrealistic or doesn't make sense. But i just thought that it was worthy of discussion and review of others. So, here it is:

Spacetime never ends. It can't end. But what defines "things" such as energy and matter, will start becoming very little amounts near the edge, fading away just like the earth's atmosphere doesnt have a set border. After the fade goes long enough that there is no matter or energy left, it's just empty space, time, gravity (which is insignificant at atomic scales), and fields. This reaches out forever. in an empty void.

Imagine you are somehow traveling through space at or very close to the speed of light and you fire a gun while you are hurtling at along at C. The bullet exits your rifle at 500 meters/second would not the bullet travel T speed of C+ 500 meters/second?

Or would the bullet not be able to exit the barrel of the gun? What about the expanding gases? Would they also be locked inside the gun.

I’m trying to work through Barbara Ryden’s book on cosmology, which is great and really only requires basic calculus. She keeps playing around with variable values and then empirical results to explain the kinds of universes there can be

Einstein’s theory has the cosmological constant even he didn’t like, and it’s not like it plays a minor role: it’s a big “push” to expand the universe.

I also read dark matter causes this. So my question is are they opening the same, or how do they relate?

Bonus question: will we ever be able to detect dark matter? And do we believe it must be quantized, and has a related field we can’t see yet, etc.?

Hi everyone, kindly bear with me since English is not my first language and I am also an amateur. So I have just watched AstroKobi's 'Could the Big Bang happen again?' video and it left me with more questions than I had before watching it.

So basically DESI was able to measure the 'growth' (?) of the Barionic Acoustic Oscillations (I envision all of this kind of like a tree's trunk growth rings) and apparently the data releases (not super good Sigma tho) says there are signs of DE weakening (?) Because the imprint of those BAOs (that are remnant of sound waves resulting from the interaction of something with photons (?)) froze when the universe changed phases and became transparent (?) and align with where galaxies like to be, and then the measurements of the galaxy clusters and filaments show a growth ever so smaller with time? So this means DE is weakening and the universe expansion acceleration is slowing down? I don't even know how they trace the size of the BAOs every x amount of time, can it be observed directly?

Among the russian doll of questions within questions I have, what strikes me the most is that those BAOs have been known for a while and there's been a mantra for 25 years or more saying the expansion rate of the universe is accelerating, there has to be a strong body of work to support this even if there was no direct measurement/observation or DESI map before, right? So I don't get this 180 now. Could you guys shed some light? Appreciate.

Curious if you find it to be legitimate or rubbish. Would love to hear your thoughts if you've studied the theory.

I’m curious about your perspective on the "Fine-Tuning" of the cosmos. When we look at the specific constants required for stars to produce carbon and for life to evolve, where do you stand?

To be clear, when I speak of "necessity," I’m not referring to mysticism or a conscious creator. I’m talking about directionality.

Think of the options as a reflection of the Anthropic Principle:

• Option 1 (Physical Necessity): Aligned with the Strong Anthropic Principle. It suggests that the universe is "wired" to eventually produce observers. Life is an inherent "drive" of the system, much like entropy has a direction.

• Option 2 (Cosmic Coincidence): Aligned with the Weak Anthropic Principle. It suggests that we only observe these "fine-tuned" constants because we happen to exist in a universe that allows it. Life is a rare, accidental occurrence in an indifferent vacuum.

Is the universe "meant" to produce consciousness as a physical necessity, or are we just a happy accident?

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If other universes exist, or if reality extends beyond our observable universe, things may be happening there that we can never measure.

But does “we can’t measure it” mean “it doesn’t exist”? Or does it only mean it lies outside the boundary of our instruments, our physics, and maybe even our concept of time?

Or maybe it IS a situation like what is north of the North Pole.

I’m curious to understand other peoples perceptions of this. This has been perplexing me for more than 50 years.

Like maybe the bang is just the first page of a book THAT WE CAN READ.

I have no formal education in astronomy or physics, just read books and watch science videos for fun, and I'm curious about the technicalities of this theory, if it's a theory actual scientists entertain at all.

My thinking is that our observable universe is not the entire universe, that our big bang is not a singular event divided from other physical events (and big bangs could be part of a black holes' life cycle), and that contraction and expansion are relative terms and can seem different whilst still being the same thing(?)(ie, if the Earth was "falling" into a sun or a black hole or whatever, but if that sun or black hole was falling into something even bigger, wouldn't it seem like the sun or black is expanding from the earth's POV, even though it's being subjected to the same process?).

If our universe, or rather, all "observable universes," are eventual/former black holes, wouldn't that mean that we are subjected to the gravity of other unseen masses/energies that exist but are too many light years away from what's observable through any form of light? If Dark Energy is not constant, if it shows signs of acceleration and/or slowing down, wouldn't it make sense if it were because other processes not different from what we see in our "universe" - stars, black holes, planets, novas, dust, etc, everything that naturally forms in our universe - are going on and changing "outside" our universe?

Some questions regarding the heat death or a similar end of the universe

Hi everyone. I am a curious student asking some many questions about our universe.

I think it contains some parts about information, entropy, cosmology, quantum physics. If you would like, I'd appreciate it if you could provide me with some explanations. Thank you in advance.

If the universe is destined to face the heat death where things will no longer be created and permanently decomposed by reaching maximum entropy, and protons will eventually decay, and the spacetime will expand so much that there is no longer gravitational force to pull particles together to support life forms, than why was the universe created in the first place, what is the meaning behind everything?

Also even if entropy is time-assymetrical, and the final result of the universe will be a dead end state, that doesn't mean the history it once beared never existed; every events from mankind to supernovae to formation if stars and whatnot were very real and existed. So where will this information of the past states if the universe be stored? Will it's history not be stored anywhere and just long forgotten and lost in the mixed sea of uniformity of the heat death universe?

Now, let's say there are external entities of the universe watching it play out to its final moment: the heat death. They are simulating it on a video game player. They have access to higher and lower dimensions of the universe, including the universe. Will such an external entity be able to predict the past states or events of the universe just by looking at its final stage and analyzing the last state of the universe, or will they only get the vast infinite possibilities of what could have been a possible history for the dead universe at best, not being able to pint point what exactly it had went through?

Another question, the time functions we have about our cosmos, will that track with heat death? Would that count as a data that specifies our universe's course of history to a certain extent?

And, unlike many physical events, can this fate never be reversed since entropy is the arrow of time and time-assymmetric? Some theories seem to suggest it is a cycle where after expanding to its limit, the universe shrinks back to a point, starting a new big bang and so on.

Now, how would external entities be able to count the time of the universe ever since it reaches its heat death state? I guess using their own clock or timer outside of the universe, right? Then how would the heat-dead universe itself show that time goes on? Even if it reaches thernal equilibrium, would it still possess movement of particles such as oscillation, since even absolute zero temperature does not mean zero energy in the quantum scale? And even if net energy transfer for work is zero, there can still be very small movements that can possibly serve as the heat dead universe's way of displaying time even at such a state?

Finally, we know that the universe has a very long life time and will continue to live longer until it reaches heat death. But once it does, the heat dead state will persist permanently. In that case, as long as the heat death is a given, can we conclude that in the grand scheme of things, the heat dead state will be much, much longer that the era when universe was capable of forming stuff in general, transfering energy, doing work, supporting life, etc? Then would the ratio of the two distinct stages time spent in years for our universe be a finite number to infinity or something like that? And are we sure that the heat death is a given, if so, how sure are we?

I am posting this question on several other science related subreddits since each may provide deep insights, so please be understanding even if you come across the same post on different communities.

Thank you very much for your replies. I really appreciate it. And thank you for your time. Have a good day.

Hello! Could someone explain what the particle horizon is like I’m 5?

I know it’s defined as the boundary of the observable universe, and it’s the distance at which light emitted at the time of the Big Bang would reach us - but I thought there was a period right after the big bang where we cannot see? Where light could not travel to us?

If the latter is true, how do we know that light coming from the particle horizon was emitted at the time of the big bang?

Thank you in advance! ☺️

I understand the Heat Death scenario but I wonder if the Great Attractor is working towards a reverse Big Bang. This would be the consolidation of matter, possibly leading to another bang, therefore raising the possibility of a recurring cycle. Are these theories mutually exclusive? Does either have a greater probability?