Cosmic expansion Definition and 22 Threads

I would like to ask a question about an interesting paper [1] back from the late 90's There, the authors propose how the universe may evolve from the near future to extremely far time scales Near the end of it (Section VI, D.), they discuss entropy and heat death: They indicate that contrary...

What is the maximal comoving distance that a probe can reach depending on its speed? If the probe travels at light speed, the maximal comoving distance that it can reach is 5 Mpc, which is called the cosmic event horizon. But what if it travels at some other speed?

https://arstechnica.com/science/2024/04/dark-energy-might-not-be-constant-after-all/ https://www.desi.lbl.gov/2024/04/04/first-cosmology-results-from-desi-most-precise-measurement-of-the-expanding-universe/ Interesting preliminary indications from DESI (which I did not know about until now)...

Do galaxies that have surpassed the speed of light during the cosmic expansion emit radiation waves comparable to the sonic boom when an aeroplane breaches the sound barrier?

I was reading this paper (*Green's functions for gravitational waves in FRW spacetimes:* [https://arxiv.org/abs/gr-qc/9309025](https://arxiv.org/abs/gr-qc/9309025)) and I had a specific question about one statement in the paper that I would like to ask: At page 6, the author says that...

https://phys.org/news/2019-01-galaxies-physics-cosmic-expansion.html can dark energy vary over the history of the universe?

Imagine that for some reason the current slow expansion of the universe is going to reverse, and the universe is going to collapse back to the pre-inflation state. [Let's make this an assumption without worrying about the mechnism] So we have a very dilute and cold distribution of energy, since...

Why should you prefer the view that cosmological expansion is due to distant galaxies moving apart according to the Hubble Law instead of cosmic space stretching, like stretching of space(-time) in the vicinity of black holes?

Why do we live in such a privileged time? arXiv:1810.10547 [pdf, ps, other] The End of Cosmic Growth Eric V. Linder, David Polarski Comments: 5 pages, 6 figures Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO) The growth of large scale structure is a battle between gravitational...

I seek a little help in understanding why we can see so far into the past to be able to view events where the light from these events should have long ago overtaken wherever the masses ended up. I get it that there was a considerable time during which the limits of the space expansion exceeded...

One way to get the universe to expand is with dark energy that pulls at the matter of the galaxy separating it or equivalently for space-time to not be perfectly flat. An alternative, in principle, would be for the gravitational pull between objects like galaxies and galactic clusters to be...

Is there any link between the energy released from the annihilation of matter-antimatter during baryogenesis and cosmic inflation or expansion/dark energy? This question came up when reading: https://www.nasa.gov/mission_pages/chandra/news/08-160.html

Hi, layman here, but hopefully reasonably educated. I did not know whether this question might fit better under relativity or under cosmology so I'm posting it here. General Relativity tells us that time slows down in gravity wells. The cosmic expansion tells us that the energy density in space...

Ive often heard that in quantum gravity space time is discrete. One analogy I've heard is to think of spacetime as being made up on space time atoms. In these models when the universe expands, is the number of space time atoms thought to increase?

According to Wayne Hu here, http://background.uchicago.edu/~whu/intermediate/intermediate.html, the universe started hot and dense and then expanded and cooled. In the hot, dense conditions of the early universe, photons were tightly glued to matter. When the universe was about 300,000 years...

Suppose that everything in the Universe - the radius of the sun, the astronomical units, radius of atoms, and everything else were to expand proportionately during the same period of time. This would mean also that the ratio of the time period of the various planets around the sun, and all local...

Would an accelerating expansion create any time dilation effects? And once the expansion velocity between any two adjacent points reached c would time stop altogether?

Hi there, This is my first post but I've been a spectator for a long time now. So I've been working on some of the basics of cosmic expansion and there is one contradiction that I came upon that I can't seem to resolve. I've looked around some of the similar threads but I couldn't find anything...

Sorry if this has been covered before, but I couldn't find anything satisfactory with a quick search. I realize there is another thread on here regarding conservation of energy with respect to expansion, but my question sort of takes a different direction; can you create energy from this effect...

Astronomers tend to presume that between any given source and observer, all light travels the same distance at the same speed. But light bends in the presence of gravity. Observe a simple prism and you will note the red wavelength bends less than violet. Forget stars and planets, how many...

I know the concept of the accelerating universe and expansion and all, but what exactly is energy density and how does it cause or explain the cosmic expansion of our universe? Thanks in advance.

I've read in books and seen on television programs that the universe is expanding, and not only is it expanding it is accelerating in its expansion. I was just reading "parallel worlds" by michio kaku, and I came across the Hubble constant, H. This number is the rate at which the universe is...