# Tag Info

Accepted

### What equation tells you how far in space you can go from a point and return?

Preparation Let $a(t)$ be the cosmic expansion factor. Let $x$ be a comoving coordinate, so two objects (like galaxies) that are separating solely due to the expansion of the universe have a constant ...
• 4,753

### What equation tells you how far in space you can go from a point and return?

sten's answer is excellent and beautifully analytical; I just wanted to illustrate the journey and show that neglecting matter (and radiation) is a good approximation. Spacetime diagram To do this, I ...
• 38.7k
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### Is Webb or any near-future telescopes like ELT capable of observing redshift changes to confirm General Relativity?

The effect whereby, as the universe expands, the redshift of an object changes with time is known as redshift drift. A galaxy at a fixed co-moving distance will have a redshift that changes with time ...
• 155k
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### Are there ways to estimate size of the "whole universe"?

tl; dr The universe is probably infinite, but if that's the case it's impossible to verify. If the universe is finite, and small enough, and the global curvature is equal to the curvature of our ...
• 15k
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### What's the name of outside the cosmic horizon?

The outside is also the universe, the inside is just the observable universe.
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### Conditions in the Early Universe

Here are some plots, calculated as described here. I assume no physics beyond the Standard Model and the concordance cosmological model. The temperature The horizontal lines are: The temperature of ...
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No, the oldest light we can see is as old as the Universe$^\dagger$$^\ddagger. This is simply because light started traveling at that time, so after one year you'd se light that had been traveling ... • 38.7k 6 votes Accepted ### What exactly is wrong with this plaque in Neil deGrasse Tyson's "back yard"? Has it been fixed? I see 2 candidates: the universe is 13.8 billion years old, so should be rounded off to 14 the cosmic horizon is 46 billion ly away • 3,034 5 votes ### How far away are objects whose light will never reach us, because of the expansion of the universe? There are several different quantities of this sort that you can define, and the definitions are fairly confusing. Hopefully the following diagram will make things clearer. ... • 3,860 5 votes Accepted ### How strong is the gravity from half of the distant universe? Not very strong at all. I get a rough figure of 3.725\times 10^{-9} m/s^2. To perform that calculation I made a few simplifying assumptions. Assume that we can ignore everything outside the ... • 15k 4 votes Accepted ### How strong is the gravitational stretch we experience from the edge of the universe? In the past, the universe was very hot and dense, and we are seeing the light from this time period as the Cosmic Microwave Background. This statement is (partially) correct. However.... Since ... • 359 4 votes Accepted ### How far away are objects whose light will never reach us, because of the expansion of the universe? You're completely correct! The farthest we can see (in principle, not in practice) is called the particle horizon. Currently, the distance to the particle horizon is d_\mathrm{P} \simeq 46\,\mathrm{... • 38.7k 4 votes ### Must time pass more slowly, relative to our inertial reference frame, inside galaxies that are currently located at half way to the Hubble Horizon? The point of the unification of space and time in relativity is that there's no sense in asking what's happening "right now" at a different position. It makes as much sense as asking what's ... • 3,860 4 votes ### Size of the whole universe if it were spherical Different data sets, analysis methods, and statistical interpretations will give different limits on the ratio \Omega between the density and the critical density. However, let's adopt \Omega=1.... • 4,753 3 votes Accepted ### What's the furthest distance that something could travel and eventually come back to Earth? There's an easy way to work out the distance to the mirror at the present time, if it moves with the Hubble flow: imagine that instead of being emitted by us, the light is emitted by the mirror image ... • 3,860 3 votes ### Size of the whole universe if it were spherical The Planck (2018) results combined with BAO constraints give a curvature density \Omega_k = 0.0007 \pm 0.0019, where the error bar is a 68% confidence limit. That means a reasonable 3-sigma upper ... • 155k 3 votes ### What's the name of outside the cosmic horizon? Since the universe is expanding, the things (galaxies, quasars, etc) that are currently in the observable region, on our side of the cosmic horizon, will someday be over the horizon. It stands to ... • 2,538 3 votes ### A black "superhole" possibility? There exists various theories, some stating that every black hole contains a new universe... In fact as a once very dense object, our universe could also be seen as a black hole itself! About what ... • 130 3 votes ### How long would a photon take to reach us if it's emitted from a galaxy receeding at c? Actually, this is a perfectly good question if interpreted in a reasonable way, i.e., interpreted as asking how long will light take to get to us if emitted at comoving age 13.8 billion years from a ... • 5,336 3 votes ### Why is cosmological event horizon closer to us than the particle horizon? The particle horizon marks the region from within which we may have received light. It started out at zero, because light from nowhere had had the time to reach us, and increased as time went by ... • 38.7k 2 votes ### Does cosmological horizon grow or decrease as it radiates? I've read that we currently believe the expansion of the universe is decreasing over time from the current ~72km/s/Mpc toward an asymptote of ~45km/s/Mpc. The size of the observable universe is a ... • 121 2 votes Accepted ### Expresion of comoving distance According to this, the Hubble constant for redshift z is H_0E(z). Meaning we need to prove that$$\int_{0}^{z_0}\frac{cdz}{H(z)} = \int_{0}^{t_0}\frac{cdt}{R(t)}$$Take the first derivative of both ... • 1,681 2 votes ### Is the universe older than 13.7 Billion years? The universe might be eternal to the past, as well as to the future. The simplest theory for that is described in "Steady state eternal inflation", formulated by Anthony Aguirre and Steve Gratton, ... • 261 2 votes ### Minimum redshift for galaxies receding$\geq c\$ when light transmitted

In spatially flat cosmologies they are the same, and your argument is one way of showing why. If the spatial curvature is nonzero then they aren't the same. For example, in the Einstein static ...
• 3,860
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### How do you calculate comoving distance and light's travel distance? According to the formulae below?

Yes, you multiply those integrals by the Hubble distance. It's like a cosmological base distance. You generally can't calculate those integrals by algebra, you have to use a numerical method, like ...
• 15k
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### Transverse Comoving distance

Angular diameter distance is the reduced circumference of the circle, centered at our location, on which the object was located when it emitted the light (or the reduced area of the sphere if you ...
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### Is the universe older than 13.7 Billion years?

When we look at objects really far away we determine the distance using the so called "redshift". One of the furthest known galaxies "GLASS-z13" sits at a redshift of 13.1. ...
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### Is the universe older than 13.7 Billion years?

We didn't determine the age of the universe by looking at what the farthest object is we can see. Instead, we looked at closer objects and determined their distance, speed and rate of acceleration. ...
• 3,034
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### Are there ways to estimate size of the "whole universe"?

So far our estimates of the size of the Universe is from what it is expected to be (i.e. calculations) rather than what we see. But there are several problems. Age of the Universe We are pretty sure ...
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1 vote

### What does black hole evaporation correspond to in the accelerating universe / black hole analogy?

The radiation from the cosmic horizon is moving inward, so the horizon should get a little bigger. That translates to the Hubble constant and the temperature getting a little smaller. If we ignore the ...

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