r/cosmology

If we follow the most commonly accepted theory for the "end of the universe", heat death, the universe will last for approximately 10^100 years after the big bang.

Given this, the stelliferous era is estimated to last up to 10^14 years, or 100 trillion years. From my understanding, that means that the habitable period of the universe should also last that long, or at least somewhere in the ballpark of that.

The thing is, we are currently at 13.8 billion years after the big bang. In other words, 13,800,000,000/100,000,000,000,000, or 0.0138% of the period that we believe that the universe will be habitable.

I understand that isn't an impossibly small percentage, but that seems unusually *soon*. How likely is it that we just so happened to evolve and pop into life at what is essentially the beginning of this period?

Could this indicate that perhaps the universe, or at least the period of habitability in the universe will end much sooner than we believe?

And yes, I know that we can't just make an assumption based on a likelihood- like if something has even the tiniest chance of happening, it probably will happen *somewhere*.

However, humanity has always assumed that we're special in some way, like being the center of the universe, and have been proven wrong time and time again. It seems a lot more likely to me that we'd be closer to an average in the universe than a crazy outlier like being the first 0.01% of life.

I have many questions about this. The hypothesis is basically that the formation of a black hole gives birth to a new universe.

The hypothesis is then expanded to include the element of natural selection, suggesting that universes that produce the most black holes will be more numerous and able to perpetuate their species.It even extends the concepts of reproduction and mutation to cosmological scales.

I think there's something problematic here. For example, what happens when black holes merge? Do universes merge? How does this work? Black holes evaporate. So do universes evaporate too?

Did Lee Smolin consider these situations? Does he have an answer to them?

I think Leonard Susskind's String Theory Landscape makes more sense than this.I'll take his side.

https://iopscience.iop.org/article/10.1088/1475-7516/2024/08/061

This paper seems to imply an odd result, namely that warp drives can cross black hole horizons, but not humanly traversable wormholes.

What are your thoughts on the current state of "finite-but-boundless" universe models. The positively curved spatial geometry case has been around since early GR, and various other topological proposals have been explored since.

Also, how does the field currently weigh these against "flat-and-infinite models" that fit observational constraints. Do you see any specific lines of enquiry that could potentially thin the herd based on the current scientific landscape?

I am also curious whether any of you lean towards a specific topology proposal over others, and if so, what makes your preference stand above other proposals?

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.

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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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.

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.

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’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.?