# Tag Info

10

This is a tricky question to answer, because it has some false and imprecise assumptions baked into it. The short version is: I don't know at what rate stuff is currently crossing the cosmic Event Horizon. But future telescopes will not see these galaxies redshift themselves into oblivion, because this will happen in the infinitely far future, literally. ...

9

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 observable universe, then we will be able to estimate its size. If the global curvature of the universe isn't positive, then the size of the universe is infinite, (...

8

The outside is also the universe, the inside is just the observable universe.

5

No, the oldest light we can see is as old as the Universe$^\dagger$. This is simply because light started traveling at that time, so after one year you'd se light that had been traveling for one light-year, after 1 million years you'd see light that had been traveling for a million light-years, and so on. There is (most likely) more Universe beyond the ...

5

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

5

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 observable universe. Assume that the observable universe is a static homogeneous sphere with a radius of 46.6 billion light-years, and a mean density of $\rho = 0.... 5 There are several different quantities of this sort that you can define, and the definitions are fairly confusing. Hopefully the following diagram will make things clearer. Z <- future infinity / \ / \ / \ D C B A B C D <- now ... 4 An analogy in the settings of the question would be a deceleration down to an expansion rate, such that eventually light from anywhere in the universe can reach us, meaning the cosmic horizon can vanish. A constant expansion rate of the universe above the speed of light would be the analogy of a black hole of fixed mass. In an infinite universe this would ... 4 Yet we also have a third axiom: There are some parts of the universe we can never observe because they are receding away from us at a superluminary speed. This is simply false, so of course it gets you in trouble if you insist on taking it as axiomatic. The average recession velocity goes luminal at redshift$z\approx 1.4$, while there are ... 4 The de Sitter spacetime is maximally symmetric, and so having a full set of Killing vector fields, it must have a static form produced by a timelike Killing vector field. One way to get without worrying about coordinate transformations is to take the$M\to 0$limit of the general spherically symmetric lambdavacuum solution, which is the the Schwarzschild–de ... 4 You're completely correct! The farthest we can see (in principle, not in practice) is called the particle horizon. Currently, the distance to the particle horizon is$d_\mathrm{P} \simeq 46\,\mathrm{Glyr}$, but as time goes on, light from more and more distant regions will reach us. If the Universe contains only "regular stuff" such as normal ... 4 The point of the unification of space and time in relativity is that there's no sense in asking what's happening "right now" at a different position. It makes as much sense as asking what's happening "right$y$" at a different$x$in Euclidean geometry. If you fix a Cartesian coordinate system then "right$y$" is mathematically ... 3 According to this, the Hubble constant for redshift z is$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 sides of your second equation to obtain, by the chain rule, and the equation (this is the definition of the Hubble constant)$H = \frac{R(t)'}{R(t)}$the ... 3 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 of the Earth and simply passes through the mirror to us. The farthest comoving distance that the mirror Earth could be is the same as the farthest present-day ... 3 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 galaxy whose comoving distance is currently increasing from us at rate c (and the Hubble law with H=70 tells us that comoving distance is 4.3 Gpc or 14 billion ... 3 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 because light from increasingly larger distance reached us. Due to the expansion it will always increase in physical size, but due to the accelerated expansion, ... 3 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 reason, then, that whatever is currently over the horizon is just the things that were once on our side, but long ago. 3 I think there's a misinterpretation here that's causing your confusion. There's a difference between the event horizon of a black hole and the cosmological horizon. The event horizon of a black hole is the place inside which nothing can escape from the black hole. The cosmological horizon, on the other hand, is the place beyond which an observer cannot "... 3 No. Space expansion doesn't slow down, unlike the expansion of cosmic horizon. That means the cosmic horizon actually swallows galaxies as they travel away from us due to space expansion, reducing their number instead of increasing it. Nevertheless, we still haven't gathered all cosmic radiation from Big Bang that occurred within our current event horizon, ... 3 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 matter's enthropy becomes, Hawking proposes that it stays on the event horizon as a holographic structure, also compared to some kind of electric hairs 3 Let's say you have a rocket near a Schwarzschild black hole. Near the horizon, the thrust required to keep stationary goes to infinity; in this specific sense, the gravitational force becomes infinite at the horizon. If the rocket is hovering above the horizon, then the difference in gravitational forces becomes arbitrarily large near the horizon. This can ... 2 No, the expansion of the universe appears to be accelerating. Hence distant objects get more and more red-shifted. Our horizon is shrinking instead of expanding. At the end, only galaxies which are sufficiently bound by gravity remain visible; this could turn out to be the Virgo supercluster. More reading about the accelerating universe on Wikipedia, and on ... 2 There are a couple of different horizons you should care about. The first is the cosmological horizon, which is the furthest you could possibly see given that photons travel at a finite speed and the universe is not infinitely old. Since nothing travels faster than the speed of light, this is quite literally the furthest we could ever hope to see - ... 2 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 gravity apparently travels at the speed of light, we should be experiencing a gravitational effect from this shell around us as well. As I understand, the ... 2 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 prefer). If the universe is spatially flat then this is the same as the metric radius of the circle. In general it's related to the radius by the function$S_k\$ ...

1

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. When you know those, you can work backwards and find a time when all those galaxies would have been in the same place, which is 13.7 billion years ago.

1

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, and available free online: It is compatible with observations of the Cosmic Microwave Background, and is centered on a Cauchy surface from which time extends in ...

1

So far our estimates of the size a 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 of the age of the Universe, 13.8B years old, and the time when the first light was emitted. This gives us a relatively good idea of the size of the Universe ...

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Holographic entropy bounds are upper bounds for the maximum amount of entropy a given region can have, rather than its current entropy--for example, the spherical region of space just large enough to enclose the Earth and its atmosphere would currently contain a much lower entropy than a spherical black hole of the same radius, whose entropy is given by the ...

1

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 fact that the radiation should cross the horizon again on the other side of the Universe, the rate of radiation would keep getting lower as the temperature ...

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