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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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What if humans doubled size… and everything else… could we notice? [duplicate]

After the big bang, everything expanded from a small mass. That expansion is said to be still happening. Imagine if everything observable constantly grew in size. EG. Everything slowly doubled in ...
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Why is $\Lambda=0$ for a flat matter dominated universe?

I was looking over some of my cosmology notes and was considering the Einstein de-sitter model of a matter dominated universe. I know that in this case that k=0 and also furthermore that $\rho(t)$ is ...
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How do we measure the age of the universe? [duplicate]

How do we measure the age of the universe if time is relative to the observer? What is the reference frame we use to measure the age of the universe?
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How big is the universe 1 second after the Big Bang? [duplicate]

In his book Astrophysics for People in a Hurry, Neil de Grass Tyson starts of with the Big Bang and narrates on what (probably) happened within the first moments of our universe. He further states (p. ...
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Continuity equation from the Friedmann equation

I know that one can derive the continuity equation: $$\dot{\rho}=3H(\rho+P)$$ from the Friedmann equations. $H^2=\frac{8\pi{G}\rho}{3}$ and $\frac{\ddot{a}}{a}=\frac{4\pi{G}\rho}{3}(\rho+3P)$ But ...
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How will the Big Rip affect chemistry and nuclear interactions?

The "Big Rip" is a theoretical scenario for the death of the universe. Let's assume that this scenario is true. What can we expect to happen to chemistry and nuclear interactions in the time ...
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Robertson-Walker metric and cosmic homogeneity

The Robertson-Walker metric is of the form $$\tag{1} ds^2 = dt^2 - a(t)^2 \Big(\frac{dr^2}{1 - kr^2} + r^2 d\theta^2 + r^2 \sin^2\theta \, d\phi^2 \Big).$$ My question is related to the $a^2(t)$ ...
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By What Mechanism Does the Universe Expand in Loop Quantum Gravity Theory?

So, the universe is expanding, and, from what I've heard, that expansion is accelerating. However, I'm unclear as to the specific mechanism involved, particularly as would be described in a Loop ...
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Why does dark energy make universe accelerate not slow down?

If mass=energy, then why doesn't dark energy not make the expansion slow down rather than accelerate seeing as gravity pulls masses together?
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Alternative values for the constant in the cosmological equation of state? (Mislaid list)

A couple of days ago I came across - but now cannot locate - an online list of about eight or nine alternative theoretical values for the constant in the cosmological equation of state (W = 1, -1/3, 0,...
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Do cosmic voids have to exist to prevent gravitational collapse of the Universe?

Since a black hole radius is 2 times the mass, it seems like any infinite distribution of matter has to have voids of increasing size as the scale changes in order for the whole Universe not to ...
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Is it spacetime or space that is expanding?

I have read answers to several similar questions but I still don't get it. Earlier explanations seem to say that laws of quantum physics and general relative are different. Let me get this straight. ...
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Doesn't expansion of space not allow a perputual motion?

Well consider the following thought experiment: We have a tool to extract/transform "nearly all" potential gravitational energy into other forms of energy. We can theoretically make such an object (...
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Looking for redshift database or data

I am looking for data wich contains the redshift values(z) of observed supernova or galaxies with their observed distanced(so the distance between the object and earth how we observe it now and not ...
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Negative work done by dark energy

If I lift a book up, I applied a force opposite its weight so there's positive work done. Dark energy is said to be pushing galaxies apart and the energy is coming from this negative work done. ...
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Does space expansion increase potential energy, and hence increase mass?

As space expands the gravitational potential Energy increases. So does the potential energy between Atoms and quarks and so on. I have read several times that these effects are so minuscule that they ...
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Areal radius, cosmology

In cosmology, what areal radius means exactly? Is defined as \begin{equation} R(t,r)=a(t) \, r \end{equation} Where $a(t)$ is the scale factor.
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Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
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Physical interpretation of FRW normal coordinates

The Friedmann-Robertson-Walker metric (I consider for notational simplicity the flat space case): $$\text d s^2 = \text d t^2 - a(t)^2\text d \boldsymbol{x}^2$$ can be brought to normal (Minkowski) ...
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Intuition for why matter dominated expansion is faster than radiation domination?

In a matter dominated universe $a_{\rm mat.}(t)\sim t^{2/3}$, while in a radiation dominated universe, $a(t)_{\rm rad.}\sim t^{1/2}$. Therefore, a matter dominated universe is expanding more quickly, ...
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Is gravitational lensing affected by dark energy?

Is gravitational lensing affected by dark energy? I mean, despite the effect if cause on the expanding Universe, could dark energy cause gravitational or anti-gravitational lensing?
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How can the Friedman Equation produce negative pressure or density?

As I understand it, the Friendman Equation provides the driving mechanism for Inflation.$$\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho+\frac{3p}{c^2}\right)$$If $\rho$ or p is negative enough, you ...
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If space expansion keeps accelerating, what would happen when the Hubble length gets smaller than the radius of a blackhole?

I am wondering what would happen if our universe expansion keeps accelerating and finally one day, the Hubble distance gets smaller than the radius of a black hole. For example, at time t, two points ...
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Is this how vacuum works?

Vacuum exerts a net zero force, or i guess pressure? on what's inside it, right? So if the universe is expanding at an accelerating rate, is the vacuum of outer space is growing? Like space is ...
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How can the red shift due to Hubble flow be consistent with the law of conservation of energy? [duplicate]

Given that there is no change in relative kinetic energy attributable to the expansion of the universe, what accounts for the difference in energy emitted vs energy absorbed (i.e. the red shift ...
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Big Bang: what is it exactly that is expanding?

I have read a bit about the Big Bang over the years, but being no physicist I have never been able to really understand what it is about. As far as I know, starting with Hubble we have been able to ...
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If the expansion of the universe were to stop, would galaxies continue moving away from each other at a constant speed?

Consider a simple universe consisting of two very distant galaxies (neglect gravity between them). The relative motion between them is such that, if the universe were not expanding, they would be at ...
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Does the universe has a center like a real explosion? [duplicate]

I don't really know much about cosmology but I was thinking one day about the center of the universe. Now 'Big Bang' was the birth of our universe. Then Shouldn't the place (or region of space) where ...
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Density and Gravity in expanding Universe

Be $a(t)$ the expansion factor and $\rho$ a density, say of an ideal gas or something. In various papers the relation $\overline{\rho}=a^3\rho$ is stated. But why should this be the case? When space ...
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Energy Equation of an ideal gas in expanding space-time

Short question: If the energy equation of an ideal plasma is written as follows: $\begin{equation} \frac{\mathrm{d}p}{\mathrm{d}t}=-\Gamma p\nabla\cdot v-\left(\Gamma-1\right)\left( \nabla\cdot q-\...
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Deriving Friedmann Equations without General Relativity

Can we derive the analytic Friedmann Equations without general relativity, starting from completely classical/nonrelativistic arguments? (If we consider sufficiently small volumes.)
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Did the big bang create an infinite number of photons?

We will always be able to see the cosmic microwave background (CMB) at about [the age of the universe] light years away. Always. Does that mean that infinite photons were created at that time? If ...
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Curvature in space time during Big Bang and present scenario

Space time in the presence of masses is curved. But during the time of Big Bang it's presumed that all the matter in this universe was at a single point, so it must have been super dense and had very ...
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What is the current value for the temperature at which Recombination took place?

I'm looking for the most recent, most accepted value for the temperature at which the photon first decoupled from the atoms during the Epoch of Recombination? That is, what was the temperature of the ...
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After inflation ended, did the expansion slow down to non-inflationary expansion because of gravity?

Assuming that inflation occurred, it accelerated from the previous expansion to inflationary expansion. After inflation ended, did the inflationary expansion instantly change into the previous ...
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How is distance between galaxies and rate of expansion of universe calculated?

I have recently read in book 'Theory Of Everything' that rate of expansion of the universe is changing 5%-10% per 1000 million years, I would like to know how such calculation has been made. Edit: I ...
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Determining the acceleration of the Universe from a single star?

It Occurs to me we might be able to find an entirely independent method of determining the Universe's acceleration using a single source. If one was to watch a single high source consistently one ...
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Where was the farthest object 13 billion years ago?

Asumme that we see a shape of the gallaxy which reached to us in 13 billion LYs. I understood that the object right now maybe in different shape and is >40 billon LYs away. But, as the universe is ...
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BAO : Relation between redshift, Hubble constant and radial

From this link https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations#Measured_observables_of_dark_energy , I can't get this relation : $$c\Delta z = H(z)\Delta \chi\quad\quad(1)$$ with $z$ ...
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Can somebody tell me what was the size of the universe and the expansion rate at the big bang? [duplicate]

I want to know the exact numbers and preferably the calculations that gives the size of the universe at the Big Bang and the initial rate of expansion at the moment of big bang. Does somebody know ...
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Cosmic scale factor $a(t)$ in a general frame of reference

In considering an FLRW type universe, the scale factor $a$ is generally indicated as being strictly a function of time $a(t)$. Isn't this only true for a comoving frame? In some other reference ...
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Travelling at lights speed of light as universe expanding

I have read that the object, the light of which is reached us in 13.8 billion years is actually about 46 billion light years away from us now, due to the expanding Universe. Lets assume that we ...
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How age of universe or galaxy estimated? [duplicate]

How scientist found age of universe as 13.8 billion years and age of Milky Way as 13.51 Billion years? Is there any proof?
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Why is the observable universe 90 billion light years in diametre? [duplicate]

The question is fairly simple, we know that the universe has been expanding for 13.8 billion years, so how did they determine the size of the universe that we can observe, and why can we not peek ...
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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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3answers
153 views

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 ...