r/cosmology • u/AutoModerator • 5d ago

Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

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u/intrafinesse 5d ago

What would be an explanation for a new force/phenomena that would explain the Hubble tension?

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u/fireballs619 5d ago

The most promising candidates involve changes at early times that reduce how far sound waves could have traveled in the primordial plasma. The reason is that we use the size of those ripples, as they are frozen in the CMB that we observe, to estimate the distance to the last-scattering surface, which depends on how quickly the universe is currently expanding. If the observed angular extent on the sky (about degree scale) corresponds to a smaller physical distance, that means it must be closer, and therefore a quicker expansion now (higher H0).

What could do this? There are a lot of proposed models. None are preferred by CMB data alone; some earn their keep once the local measurements of H0 are brought into the mix. Some popular ones include early dark energy (i.e. some early time field that behaves like dark energy does today, briefly increasing the expansion rate at early times and reducing the sound horizon size), or a time-varying electron mass. The latter works by allowing neutral hydrogen to form at higher temperatures, meaning the plasma "shuts off" at an earlier time, so the snapshot of the soundwaves we see haven't had as much time to travel as they otherwise would. Relatedly, another possibility is that something causes baryons to have excess clumping at early times. This also allows neutral hydrogen to form sooner and the sound waves to shut off sooner. Primordial magnetic fields could generate the forces required to clump the baryons up.

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u/AstroEducation 5d ago

Assuming it is confirmed, it would require some type of new physics beyond the LambdaCDM model. Some of the leading ideas is Early Dark Energy, time evolving dark energy, or other more exotic types of physics. Right now there many more models than observational constraints so we would need more/better data to start discriminating among the models.

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u/Main-Blacksmith944 5d ago

If there was no eternal universe before the Big Bang does that mean, that the Big Bang was the beginning of time, space and matter?

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u/Vast-Control4452 5d ago

It's not understood. Currently the answer is "time didn't exist before the big bang." So yes the BB was the beginning of spacetime and matter/energy.

But you're REALLY pushing to the edge of human understanding and the data and evidence are scarce. It was so so long ago.

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u/thebigeverybody 5d ago edited 5d ago

So yes the BB was the beginning of spacetime and matter/energy.

The person asking the quesiton thinks the Big Bang theory says there was nothing before the Big Bang and the universe couldn't be eternal.

Isn't it more accurate to say that The Big Bang only says this iteration of the universe began, saying nothing about what did or did not exist before? And it doesn't say the the universe couldn't be eternal?

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u/Vast-Control4452 5d ago

There is no "before" like there is no North of the North Pole.

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u/thebigeverybody 5d ago

Am I misunderstanding this?

Was the Big Bang the origin of the universe?

It is a common misconception that the Big Bang was the origin of the universe. In reality, the Big Bang scenario is completely silent about how the universe came into existence in the first place. In fact, the closer we look to time "zero," the less certain we are about what actually happened, because our current description of physical laws do not yet apply to such extremes of nature.

The Big Bang scenario simply assumes that space, time, and energy already existed. But it tells us nothing about where they came from - or why the universe was born hot and dense to begin with.

https://lweb.cfa.harvard.edu/seuforum/faq.htm

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u/Vast-Control4452 5d ago

Sure, but I think OP and Harvard are talking about different times. OP is referencing The BB as the "time zero" while Harvard is describing the rapid expansion that is technically the BB.

Sort of like someone unfamiliar with evolution thinks it goes back to the origin of life, evolution doesn't explain the origin of life, just the diversity of life.

Technically the BB is the rapid expansion, but many layman use it as the starting point of the universe. They are extremely close though.

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u/thebigeverybody 5d ago

But Harvard (and other scientists) aren't saying there was a "nothing" before the initial singularity, right? This is because nobody knows if the initial singularity came into existence, right?

OP thinks The Big Bang says that nothing existed previously and the universe can't be eternal, which proves their god. Ignoring their conclusion, my understanding of the Big Bang does not say either of these things because we just don't know.

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u/Vast-Control4452 5d ago

How many times can I say that "before the singularity" isn't a valid statement.

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u/thebigeverybody 5d ago

Which means The Big Bang theory doesn't say nothing existed before it, right?

This is because we don't know if the initial singularity had an origin, right?

Which means if someone thinks the Big Bang says there was nothing before, they're wrong, correct?

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u/Vast-Control4452 5d ago

Like I mentioned in my other comment, the big bang is the period directly after the singularity. Not the singularity itself.

If in YOUR definition of the big bang, you're including the singularity, which scientists don't, then you can't say anything came before it.

If you use the technical definition, the singularity came before the big bang. But nothing can come before the singularly because time didn't exist in the singularly.

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u/Careful_Broccoli4852 23h ago

But it cant be eternal. If it were, we would be in thermodynamic equilibrium.

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u/--craig-- 5d ago

As worded, the answer is no.

There are multiple possibilities. Time might've begun 13.8 million years ago, some longer period or it might not have a beginning.

Matter was created a fraction of a second after the big bang.

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u/violentbowels 5d ago

Matter was created a fraction of a second after the big bang.

From 'nothing' or from preexisting energy/matter?

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u/--craig-- 5d ago

Matter can be created from pure energy. It can't be created from nothing.

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u/violentbowels 5d ago

Ok, that lines up with my, very limited, understanding. The energy was already there or was what expanded and the expansion caused it to transform from pure energy into energy and matter.

Is that at least fairly accurate?

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u/--craig-- 5d ago edited 5d ago

We don't fully understand where the energy came from but matter and anti-matter pairs are produced from pure energy all the time, both of which carry energy. We also don't fully understand why we have more matter than anti-matter.

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u/violentbowels 5d ago

Got it. Well, kinda. I appreciate it.

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u/Main-Blacksmith944 5d ago

Does the Tolman Law and borde-guth-vilenkin theorem refute cyclic models?

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u/--craig-- 4d ago

If they hold, which we're not sure about yet, then they would preclude traditional cyclic cosmology, but not all possible cyclic models.

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u/OverJohn 4d ago

BGV doesn't refute cyclic models - specifically BGV applies to spacetimes which have external expansion everywhere in the past, but traditional cyclic models have alternating expansion and contraction in the past.

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u/FakeGamer2 5d ago

If Eternal inflation is true, can any given individual bubble die due to heat death and then stay that way for eternity? Or does it strictly depend on if there is a lower vacua to tunnel to?

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u/--craig-- 4d ago

False vacuum decay would cause an entropy increase so wouldn't reverse heat death. Independent of that, heat death is only a statistically certain fate so given enough time it's actually reversible by quantum fluctuations alone.

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u/Left-Fruit-5688 1d ago

want to ask something with no prior science knowledge I’m 15 currently so not even done with basic education but anyways here’s my theory. Since energy can’t be completely destroyed and time is kinda infinite doesn’t that mean that in a sense an infinite or finite amount of new universe will exist after heat death because given enough time energy will find its way and I also believe our existence makes it very likely for this to be real. So maybe multiverses don’t co exist in the same time but in a linear kind of way and the only way to access is through time travel and that why no one visited our universe because it would create a paradox. It’s probably dumb but I want to know what you think about it.

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u/--craig-- 1d ago

want to ask something with no prior science knowledge I’m 15 currently so not even done with basic education but anyways here’s my theory. Since energy can’t be completely destroyed and time is kinda infinite doesn’t that mean that in a sense an infinite or finite amount of new universe will exist after heat death because...

I can follow what you're trying to say until this point but after that I'm just getting word salad.

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u/RichardRickyRick 1d ago

When we look at the universe, we see it as it was. Based on how far away a galaxy is from the earth, the light from that galaxy takes a great deal of time to reach us. So each galaxy appears in the spot it was, not where it is.

But what does the universe look like at this very moment? Where are the galaxies now? How many of the galaxies we see still exist? Are there new galaxies? Do these questions even make sense and do the answers to them have any meaning?

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u/--craig-- 1d ago edited 1d ago

It does make sense. There is the concept of cosmic time, the coordinate we use for the big bang. You can use this to ask your questions, circumventing the problem of the relativity of simultaneity.

To answer your questions, you'd need to project what we see forward in time. Distant galaxies would be more mature and be further away than they appear. Many would have merged and many would have become dormant.

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u/Careful_Broccoli4852 23h ago

What is halo entropy and how does it help constraint dark matter models? Cant we just constraint dark matter haloes by observed densities, e.g NFW?

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u/--craig-- 15h ago edited 13h ago

Dark matter is modelled as a collisionless gas. The energy distribution of gas particles tends towards the one with maximal entropy.

The NFW distribution is empirically determined from n-body simulations and doesn't represent maximal entropy.

It's not yet clear what the cause of the discrepancy is.