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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Why is the Baryon Acoustic Oscillations (BAO) calculated like this?

In the Wikipedia, it says that, when calculating the Baryon Acoustic Oscillations (sound horizon), we measure $150\text{ Mpc}$, saying that the sound horizon is the "Physical Length of sound horizon ...
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Is inflation a possible explanation for what started the expansion?

Alan Guth stated in this article But let me start the story further back. Inflationary theory itself is a twist on the conventional Big Bang theory. The shortcoming that inflation is intended to ...
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Is information conserved in an expanding universe?

From the classical and quantum mechanical viewpoint, is information conserved in an expanding universe or not?
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Can universe still expand without energy and matter?

I heard dark energy is the intrinsic property of space and it cannot be particle because particle's density dilute when volume increase, I do not know how nothingness can have property and Einstein ...
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Did the rapid inflationary expansion slow down a lot because the inflaton field had decayed, or because of gravity from matter/radiation?

What happened to the rate of expansion right after inflation ended? Did the rapid inflationary expansion slow down a lot because the inflaton field had decayed, or because gravity from matter/...
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Relative size of galaxy calculation

So this is a past exam problem, however I am confused to a question which is related to this graph and the questions states: Question: On the graph, one galaxy is labelled A. Determine the size of ...
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From the expansion to temperature and recovering equilibrium number densities from the scale factor

For a particle species of mass $m$, when the temperature $T$ of the Universe is $T>m$, the equilibrium number density $n$ falls as $$n\propto T^3\tag{1}$$ and for $T<m$, it falls as $$n\propto ...
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How does calculating Hubble time work?

For calculating we take $t=\frac dV$, where $V=Hd$ . But velocity isn't uniform for an object as distance also increases with time. And if velocity isn't constant the equation $t=\frac H d$ also isn't ...
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Why Empty universe have to obey the Negative Curvature? [duplicate]

For empty universe it seems to me that we can have two solutions. $$H^2=\frac {8\pi G\epsilon} {3c^2}-\frac {\kappa c^2} {R^2a^2(t)}$$ For an empty universe when we set $\epsilon=0$ we get $$H^2=\...
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Why is the heat death, rather than a Big Crunch, the most accepted theory of the ultimate fate of the Universe?

The Heat Death is accepted by most as the end of the Universe, but how can that be? Wouldn't the Big Crunch make a lot more sense? I mean, even if everything in the Universe is spread out uniformly ...
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Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
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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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Time slowing down vs. universe expanding

Einstein said that it is impossible to distinguish between the effect of gravity and acceleration (so if you stand in an accelerating elevator in space it would not feel any different than if you were ...
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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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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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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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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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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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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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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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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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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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How far has a 13.7 billion year old photon travelled

I've read that the size of the observable Universe is thought to be around ~46 billion light years, and that the light we see from the most distant galaxies were emitted ~13.7 billion years ago as a ...
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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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Is the cosmological redshift caused by the Planck mass increasing?

The standard explanation for the cosmological redshift is that photons emitted from far away galaxies have their wavelengths lengthened as they travel through the expanding Universe. But perhaps the ...
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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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If photons don't “experience” time, how do they account for their gradual change in wavelength?

It is often said that photons do not experience time. From what I've read, this is because that when travelling at the speed of light, space is contracted to infinity, so while there is no time to ...
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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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Is the universe actually expanding?

Up until recently I was fairly sure that the universe is expanding, i.e. the (spatial) metric is changing proportionally to the scale factor, such that the distance measured between objects is ...
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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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Space expansion and speed of light

I recently saw a video on gravitational waves that says that expansion of space can only be measured due to changes in speed of light as everything else that could have been used to measure the ...
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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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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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How much has the universe expanded in total from the onset of inflation until today?

Starting from the onset of inflation, through inflation, reheating, radiation domination, matter domination, the transition toward dark energy domination all the way to today, how many e-folds of ...
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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 ...