I’m sitting at my desk, visualizing Earth’s rotation around the Sun. I can see us from the Sun’s perspective. I get that our star is one among billions orbiting the galactic center. If we picture Earth being dragged by the Sun around the galactic center, which direction is Earth’s forward progress? Towards our North Pole or towards our South Pole?

For the sake of argument imagine this scenario as a time-lapse spanning 100 million years.

This is probably a question that Google could answer for me, but I want Reddit-scientist answers.

I was having a conversation with my girlfriend about how awesome black holes are and the phenomena behind them. A general, likely dumb, question is - they destroy matter instantly in their event horizon. No matter, as far as I know, survives when it gets sucked in. But they have a gravitational pull like no other, which is that gravity is created by mass, which mass must have some material to build mass, no?

I guess what I'm confused by is that they have insane gravitational pull, yet destroy any material that comes in contact with them due to their billions of pressure/pull. Yet, they gain size. They gain mass, creating more gravitational pull. What is that mass made out of? Is that the question that scientists are trying to understand as well? Is it "dark matter"?

Thank you for any help understanding this, me and my girlfriend will read answers together :)

Re Dark Matter. Rounding figures off. ‘If’ ( a big if) Dark Matter is proven not to exist, does the 25% of the Universe made up of Dark Matter then need to be redistributed to Ordinary matter. Is the 25% added to Ordinary matter and Ordinary matter is then said to make up 30% ofthe Universe? Or…does the percent of Dark Energy increase?

Note: I know this is a generalization but just trying to get perspective.

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Hi! Maybe that's a noob question but I'm having trouble understanding something. In recent studies presenting the timescape model, scientists claimed that dark energy could be an illusion due to time dilation of different areas of the universe due to varying density.

I think I had a misconception about dark energy, because I thought that it was responsible for the expansion of the universe, but I see now that it should be responsible of the acceleration of its expansion only.

So... what makes the universe expand in the first place?

What are some of your favourite cosmology books?

I read in this article that the KBC supervoid could be causing Hubble tension because the mass around the void causes the mass inside in the void to flow outwards, adding to the hubble constant when calculated using the observation method. Isn't this really simple to test? Like, can't you just create a model of our universe and test the effects or something? Or has nobody tested it yet because of something else I don't know?

https://www.space.com/the-universe/hubble-trouble-or-superbubble-astronomers-need-to-escape-the-supervoid-to-solve-cosmology-crisis

Hi all,

I have a question I can't figure it out for a long time.

So, we have so called vacuum that creates virtual particles due to a tunnel effect. We call it "virtual" just because these particles interfere with its own anti-particle and return its energy to vacuum. That's why we can't catch them unless we are in nearby blackhole. That's clear for me so far.

And I have a questions that annoying me:

We know that virtual particles are born on the scale that is much less that real particles exist. So in my opinion, every real particle (e.g. electrons, quarks etc) should be surrounded by born of vacuum "virtual" particles. every single moment and every single time, That's why I suggest that real particles should interfere "virtual" particles before it goes back to vacuum. And this interfere should destroy our world because electrons should leave their orbits, quarks should change their spins etc. But we don't observe this, so what should happened to avoid this situation?

Thanks in advance.

Consider a binary pair of black holes spiraling towards each other as gravity waves take away their energy. Assuming they formed together, they would have the same sense of rotation and revolution around each other.

As the holes approach, the first collisions would between the accretion discs of each body. Would this not be like a cosmic particle accelerator and might there be a detectable signature?

Second, there is frame dragging with each black hole. As with the accretion discs, the directions of dragging will be opposite in the region between them. Whan effect would this have on spacetime? I envision a vortex of spacetime with extreme properties.

Finally, when the event horizons merge, there will be a short time where there will be a region in the overlap zone where a particle within it has TWO singularities in its inevitable future. How is this resolved and would the singularities merge at near light speed?

Thanks.

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I’ve skimmed over a few popular cosmology textbooks and typically, despite being so fundamental, the Boltzmann equation is usually just presented over the course of a paragraph then used for the rest of the book. I tried to find a statistical mechanics book that covered it more in depth but I found no mention of the form of the Boltzmann equation used in cosmology (the one with the (f3f4-f1f4)|M|^2 term in the collision integrand). I’m interested in seeing a derivation/more thorough discussion of it but this is proving to be quite challenging. I’ve seen the classical case presented in some books (like Reif) but never the quantum case. Any references would be appreciated

The comoving distance is defined to be constant for the comoving observers.

Distance measure on wiki:

The comoving distance d_C between fundamental observers, i.e. observers that are both moving with the Hubble flow, does not change with time, as comoving distance accounts for the expansion of the universe.

(...)
Comoving distance factors out the expansion of the universe, which gives a distance that does not change in time due to the expansion of space (though this may change due to other, local factors, such as the motion of a galaxy within a cluster); the comoving distance is the proper distance at the present time.

Why the comoving distance doesn't change with time if it accounts for the expansion and is presently also equal to the present proper distance? The latter obviously changes with time and is also the result of the expansion. The value of the present time t_0 changes with the flow of time and both the proper distance d(t) and the comoving distance χ change with it because they are equal at the present time with the scale factor a(t_0)=1 due to their relation d(t)=a(t)χ.

Comoving and proper distances on wiki:

Comoving coordinates (...) assign constant spatial coordinate values to observers who perceive the universe as isotropic. Such observers are called "comoving" observers because they move along with the Hubble flow.

How can the comoving observers receding away with the Hubble flow have constant spatial comoving coordinates assigned, if their comoving distance continuously increases with the Hubble flow in (t_0, ∞) time range?

Am I right, that the comoving distance doesn't change in the past time in range (0, t_0) for a(t)<1 but it definitely changes in the future time in range (t_0, ∞) for a(t)>1? In that case the statement that it doesn't change with time would be half correct.

If passing moment stretches over the whole present cosmic time/epoch with undefined timespan, then in every passing moment we fix the comoving distance for the whole past at the new value equal to the present proper distance for the needs of all the calculations that use their relation d(t)=a(t)χ. By "we" I mean us and the future astronomers living millions or even billions of years from now.

This qualitative animation shows how the comoving distance is both constant for the past and increasing with the expansion. You can imagine that a single frame of this animation takes 1 mln years, so there is 1 frame per 1 mln years. t_0 does not change in a single frame interval and the comoving distance remains constant with it for the same time.

Example: The comoving distance is χ=1 in arbitrary units of length. The scale factor a(t)=1 now as well as in the far future, because the future astronomers will also normalize a(t) for their convenience. The present proper distance will not be the same with the future proper distance. We have d(t)=a(t)χ=1 today and they will have d(t)=a(t)χ>1 in the future, but because they will also set a(t)=1 for their "now", their comoving distance χ>1, so χ has increased with the cosmic time that has passed between our "now" and their "now" due to their normalization of a(t).

PS. I understand, that top 1% commenter must remain top 1%, but I regret the fact that the bottom 1% must remain bottom 1% on the occasion. My comments are downvoted only because my reasoning stands in opposition to the comoving distance definition.

Just a bit of speculation and questioning why something does or does not fit the requirements to win a Nobel prize.

Not to detract from the importance of the photoelectric effect, but maybe I personally feel like general and special relativity were revolutionary concepts and discoveries, and kinda underpin a lot of how our universe functions at the largest scales.

There’s more I could say about how amazing relativity is, but I think you guys get the picture.

Hey, biology student here who is interested in cosmology!

I do have some understanding of things like quantum mechanics but that too only with scientists explaining it and they mostly dumb it down to layman terms so the average person can understand.

I first need to brush up on some physcis coz I studied it only for about 2 years in high school.

So to put it in simple words I want some books that will help me learn more about cosmology, quantum mechanics and theory of relativity.

When we consider the collision term, say for a process 1+2<->3+4, we have an integral with a factor of (f3f4-f1f2)|M|^2 δ^4 (neglecting blocking/enhancement factors) over the momenta of 2,3,4, with the δ^4 balancing out momentum/energy. Since we don't have an integral over p1, the integral is "asymmetric" and makes the f3f4 term near impossible to evaluate. However, if f3,f4 follow a Maxwell distribution, we have f3f4=exp( (mu1+mu2-(E3+E4))/T )=exp( (mu1+mu2-(E1+E2))/T ) which allows us to integrate over |M|^2 δ^4 to use the cross section of the process.

If we can't assume this, it seems like the best we can do is a 6 dimensional integral. Am I being stupid or is this actually the best we can do? Is the only feasible way to then evaluate this through methods like Monte Carlo integration?

Haven't see this posted here yet, so I wanted to share it and get's folks thoughts about it. Feels like a 1-2-3 gut punch for dark energy this year: JWST independently verifies the Hubble Tension, DESI papers take another hit at the cosmological constant, and then this paper right before Christmas.

Thoughts?

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Cosmologists seem to agree nowadays that the expansion of the universe is accelerating. I believe observations from the Hubble telescope were showing this first (https://science.nasa.gov/mission/hubble/science/science-highlights/discovering-a-runaway-universe/).

Does this mean that looking backwards, expansion must have gone more and more slow?
And if so, does this mean that we might have underestimated the age of the universe?

Detection of the Cosmological Time Dilation of High Redshift Quasars
https://arxiv.org/abs/2306.04053

The Dark Energy Survey Supernova Program: Slow supernovae show cosmological time dilation out to z∼1
https://arxiv.org/abs/2406.05050

https://preview.redd.it/wdbw5fh3yx6e1.png?width=515&format=png&auto=webp&s=29637e4db81bbfd493fd2a9870ddfa5a5e20faf2

Commonly accepted metric of the expanding spacetime is the FLRW metric, but it doesn't take cosmological time dilation into account even though the time dilation is the expansion of time. Photon wave's period extends by the same factor as its wavelength, but the FLRW metric describes the latter without the former, so how can it be a correct description of the expanding spacetime?

When we calculate the observable universe radius using FLRW metric we set 0 for the proper time, because it doesn't flow for a photon. This simplifies the metric to the equation a(t)dr=cdt. We divide both sides by a(t) and integrate it to get the radius r. Scale factor is applied only to the expanding space and we calculate the observable universe radius from it. How can this calculation be correct if it's missing cosmological time dilation CTD?