Questions tagged [space-expansion]

Space expansion is a cosmological phenomenon wherein the proper distance between two spatial points for a given inertial reference frame increases from one moment of time to another. That is, space itself expands; the added distance is not due to relative motion of points or objects.

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How can the effects of dark energy be distinguished from the effects of the acceleration of the universe from the big bang?

This is a conceptual question and has probably been asked before in many guises and forms. Mine has a little twist to it. I realize the Hubble Law tells us the farther back in time and space we look ...
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Why can we see the cosmic microwave background radiation?

This radiation (CMBR) is said to have its origin at the surface of last scattering that exposed itself when the big bang universe had expanded for less than a million years. In order to see radiation ...
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Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?

In Newtonian gravity, an infinite volume filled with a uniform distribution of mass would be in perfect equilibrium. At every point, the gravitational forces contributed by masses in one direction ...
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Why does inflation end when the potential becomes too steep?

Inflation ends when the potential becomes too steep. Is it because kinetic energy increase when the potential becomes steeper?
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Is the universe ~14 billion years old or that's the farthest photons which reached Earth?

I know that the universe: It's around 13.772 billion years old It expands But it's not clear to me if this is not merely the age of the farthest known photons which reached Earth.
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Fraction of the Universe that is unobservable

I was watching this talk by Leonard Susskind. Near the end of his talk, he mentions how, because of the accelerating expansion of the Universe, there is a horizon to the Universe beyond which we ...
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Cosmological redshift : “time of emission” vs “during travel”

Let’s consider a very distant galaxy, suppose it has no peculiar velocity, and let’s assume space expansion; If I understand correctly there would be a redshift due to the value of space expansion at ...
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How is red shifting explained with quantum mechanics? [duplicate]

Light travels as waves through space, and since space is expanding, the light gets stretched out. This is the explanation for red shifting I learned before. Now I know some more and that explanation ...
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Cause of observed galactic red shift or acceleration in expansion of the universe: velocity vs gravity.

Scientist look out at the universe and observe red shifts for galaxies that are roughly proportionate from their distance from us and attribute it to a velocity away from us. Also there is an ...
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What happens if you travel through space at the same rate that space expands?

Hubble's Law gives the rate at which the universe is expanding, as a function of distance. Take an estimated value for $H_0$ of 70 (km/s)/Mpc. If we take some star 1 Mpc away from my current position,...
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Is the expansion of the universe really accelerating? Or does it just look like it?

I pulled this quote from an article on the Hubble Constant: "...for example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a ...
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Will the Hubble law be the same no matter where I am positioned in the Universe?

If I am measuring the expansion from earth I would find the Hubble Law to apply. If I switch positions and locate myself to a far distant galaxy and then measure the expansion I would also see the ...
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How do we know the age of Cosmic microwave background? [duplicate]

How do scientists know the age of the CMB? And how does it correlate to the age of universe?
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Does the unobservable universe exist in the present?

Is our observation about universe limited due to the incapacity of telescopes to look further or is it just because we look too further into the future, so the "unobservable" universe hasn't been ...
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How can we be sure if Universe has exploded with big bang? Can't it be implosion? [duplicate]

Why do we say the universe has exploded in the event of big bang? When we talk about big bang there was no concept of outside with the absence of space, so should’t the universe be imploded into the ...
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How can the universe expand faster than the speed of light? [duplicate]

In Neil deGrasse Tyson's Astrophysics for People in a Hurry, he writes: By now, one second of time has passed. * The universe has grown to a few light-years across... If nothing can move ...
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Is the cosmological Continuity Equation an expression for energy conservation in General Relativity?

Is the cosmological continuity equation $$ \frac{d\rho}{dt} + 3H(\rho + p/c^2) = 0 $$ an expression for energy conservation in General Relativity?
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Is all matter within the observable universe?

The observable universe is everything within the range of the distance light can travel since the beginning of the universe. Since relativity states that matter cannot travel faster than light in any ...
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Seeing the Big Bang [duplicate]

The Big Bang happened almost 14 Billion years ago. Theoretically speaking , if we could teleport (Instant teleportation) to somewhere that’s 20 Billion light years away from the Earth , if we looked ...
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Big Crunch vs Big Rip vs Heat Death vs Conformal Cyclic Cosmology… What's the Most Common View? [closed]

I just saw that Penrose recently published a paper on Conformal Cyclic Cosmology and it made me wonder, what exactly is the consensus view these days? I first heard of Penrose in connection with Orch-...
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What is this metric's scale factor?

While answering this question about a hypothetical 3-sphere universe $S^3$ expanding with a constant acceleration $\phi$ from a zero initial speed $$ r=\dfrac{\phi}{2}t^2$$ I started from a generic ...
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Will everything eventually be a single black hole? [duplicate]

Given the idea that gravity is always a factor (even though greater distances are excessively tiny), would everything eventually end up forming a single black hole? My thinking is that, with infinite ...
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What happens when cosmic background microwaves enter a bound system?

Gamma radiation after spending eons in expanding space have now “stretched “ into microwaves. What happens when these waves enter a bound system like a galaxy or galaxy cluster. Expanding space ...
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How do we know normal expansion without inflation is not enough to dilute the density of magnetic monopoles?

Assuming magnetic monopoles was produced in the early universe. However we don’t see any of them. Wouldn’t expansion even without inflation dilute their density enough that they are unobservable?
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Mass and Expanding Space

1) space is expanding at an increasing rate, therefore things are getting farther from each other and therefore increasing in velocity. 2) the faster an object moves relative to another, the more ...
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Experimental proof of space expansion

We know the universe expands with the space expansion. This follows from the FLRW solution that nicely explains the observed Hubble flow. (Since any existing acceleration is extremely small, let's ...
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Is the space between galaxies that are close to each other contracting? [closed]

The Milky Way and Andromeda are moving towards each other as indicated by blueshift. Are they just moving towards each other through space or the space between them contracting?
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Is space expansion causing increasing distances between galaxies?

I read/heard that the expansion of the universe would lead to any observer not being able to see other galaxies as time goes on, due to their light not reaching them anymore. But there's something I ...
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Is the universe a giant telescope?

Due to the space expansion, the universe should act as a giant telescope. The farther objects are from us (beyond the redshift of $z\approx 1.5$), the larger (not smaller!) they should appear in the ...
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Was the universe already expanding before inflation occured?

Was the universe already expanding before inflation occured or did inflation cause the universe to start off expanding? By “cause it to start off expanding” , I mean the cause of the initial ...
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Density parameter in matter-dominated universe

With regard to the density parameter derived from Friedmann Equations which is: $$ Age = D_H\int_{z}^{\infty}\frac{1}{(1+z)\sqrt{\Omega_R(1+z)^4 + \Omega_M(1+z)^3 + \Omega_K(1+z)^2 + \Omega_L(1+z)^{(...
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Does Friedmann fluid equation prove that energy of the fluid is not conserved?

The Friedmann equation in question is:$$ \dot\rho + 3H \left(ρ + P\right) = 0 \,.$$ I’m a layperson so I don’t know why this equation is sometimes called fluid equation, energy conservation equation, ...
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Trying to reproduce curves with angle of CMB anisotropies as a function of distance and curvature parameter

I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure: For this, I don't know what considering as abcissa (comoving distance ?, i.e $$D_{comoving} = ...
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What is $\tau$ and $\sigma$ in a geodesic $ψ(x) = (\tau(x),\sigma(x))$?

I'm trying to replicate the results of this paper (Eternity in 6 hours, written by Stuart Armstrong and Anders Sandberg). On pages 14-15, it discusses the path of an intergalactic probe that is ...
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Would eternal inflation be really eternal since quantum fluctuations are usually very rare and very small?

I’ve read somewhere that quantum fluctuations in the inflaton field are usually very rare and very small. If the effect of quantum fluctuations pushing the potential uphill is too rare and too small ...
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Is the universe expanding?

If spacetime is curved by object with mass, just like the sun curves space time and so makes the planets orbit around it and will eventually in time make them crash towards it then shouldn't the ...
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Is there any way to tell if the Big Bang happened everywhere or just in some regions of the entire universe?

On large scales, matter seems to spread uniformly in our observable universe so we think the Big Bang happened everywhere in the observable universe. Is there any way to tell if the Big Bang happened ...
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Expansion of universe, distance confusion

I am having trouble understanding expansion of the universe. Say my coordinate system uses atomic clock cycles for time and the number of clock cycles for light to bounce back to me for distance. Say ...
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How could the universe's expansion “remove the mean gravitational potential”?

I'm citing W. C. Saslaw's The Distribution of the Galaxies: Gravitational Clustering in Cosmology, chapter 25, where he adresses (what seems to be a Newtonian approximation of) the thermodynamic ...
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Does object from which the universe is created still exist in its original form? [closed]

If time is passing more slowly near objects with bigger gravitational mass, could it be that the time at the object from which the universe is created has not started yet (or very little of time has ...
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How do we know the universe is expanding and wasn't already infinite?

From what I know, the decision that the universe was expanding came from the discovery of the redshift and that other celestial objects, such as galaxies, were moving away from us; the farther they ...
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In what sense the scale factor $a(t)\to 0$ lead to the big bang singularity?

How do we understand that the limit $a(t)\to 0$ (where $a(t)$ is the scale factor) lead to a spacetime singularity? Is it by substituting $a(t)$ in the FRW metric and concluding that space makes no ...
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How was it possible for the big bang to occur?

I think the person who asked this Phys.SE question meant a different thing than any of the people who wrote an answer thought so I'm asking what I'm guessing the author probably really meant. If there ...
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Alexander Vilenkin - The Universe probably had a beginning [duplicate]

Is it true that the Universe can not have an eternal past? It seems like Alexander Vilenkin tries to argue this? Does this imply that the Universe had a beginning? Link to his publication regarding ...
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Is stellar luminosity vs redshift calculated incorrectly?

In this paper the author, T.X. Zhang, asserts that the luminosity of distant supernovae normally is calculated incorrectly. His point (if I understand his paper) is that "luminosity" should mean ...
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How far back in cosmology is there observational evidence?

It seems "the first three minutes" are well established... back to the first second? 10 seconds? What about the "hadron epoch" going back to the first microsecond? Or before that? I know there are ...
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Did negative pressure cause the universe to start off expanding at the big bang?

What caused the universe to start off expanding in the first place. Was it negative pressure that drove the initial expansion or was it just something else?
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Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion?

I have a question about inflation theory. Why do we think that the energy density of false vacuum/scalar field doesn’t dilute with the expansion? Could it be that the energy density does dilute with ...
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How do you explain cosmological red shifting in terms of gravitons?

We know that the photons from the big bang are continually being red shifted and losing more and more energy. In terms of the graviton view, how would you explain that? Where is the energy going? Are ...
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Explicit equation for scale factor - how long valid?

In “The picture of our universe: A view from modern cosmology” (https://arxiv.org/abs/astro-ph/0209504 - eqn 63, page 26) there's an explicit equation for the scale factor for a flat universe with $\...