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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 is the relationship between the Cosmological Constant and the Cosmological Principle?

I believe I've misunderstood a relationship between the cosmological constant and the cosmological principle having read: Einstein introduces his cosmological constant which attributes, in the ...
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Volume of dark energy in space [duplicate]

As I understand it, Dark Energy is believed to reside in empty space. As the Universe exapnds, more empty space exists inside the Universe and so the small energy per unit of empty space adds up to a ...
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The Expansion of universe [duplicate]

if the space itself is expanding at considerable rate, that means we are also expanding along with it(as we are the part of the universe). so why can't we feel ourselves expanding or detect it?
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Does an atom or proton (cosmic ray) get stretched when traveling through expanding space?

I have read this question: Why does space expansion not expand matter? I do understand that matter inside galaxies will not get stretched because inside galaxies, gravity dominates over dark energy. ...
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Cosmological Inflation: If Photon expands, why not other matter?

I keep repeatedly reading in many Stack Exchange and Quora questions that space of universe expands but particles (matter) don't, see e.g. this Phys.SE post. The reason given is that particles are ...
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Correction to Newton's law of Gravity for dark energy

Is there any correction to Newtons law of Gravity that takes into account the expansion of the universe?
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Could one theoretically use the expansion of the universe to travel through it? At least in one direction?

Could one theoretically use the expansion of the universe to travel through it? At least In one direction? That’s it that’s my question. I’m not a physicist but I do get ideas. I also wonder if one ...
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Where‘s the coordinate origin of the universe? [duplicate]

Given all mass originated from 0/0/0, what point do we have approximately? In km
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How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?

How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?
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Why don't we see galaxies going back in time?

Galaxies 4,200 megaparsecs away will travel faster than the speed of light away from each other, so why don't we see those galaxies as Going back in time up to the moment of the big bang?
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Is the universe be expanding at different rates depending on its temperature and density?

As I understand it, when observing different regions of the universe we get a different rate of expansion (the two Hubble constants), specifically the more distant observation seems to return a lower ...
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Is Hubble's Law Only Valid Nearby?

I am looking this paper by LIGO, and it claims at the very beginning, At nearby distances $(d \lesssim 50 \text{ Mpc})$ [the Hubble constant] is well approximated by the expression $$v_H = H_0d, \...
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Existence of things outside the observable universe

I've read multiple physicists talk about stuff (e.g. galaxies, black holes, etc.) "existing" outside the observable universe, but it's unclear to me in what sense these things "exist". By definition ...
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Is universe symmetric about a point?

We have a good amount of discussion and theories on the formation of universe. I want to ask is universe symmetric about a point? I think that the answer should depend upon the uniformity of ...
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Is space itself expanding or is it just momentum from the Big Bang carrying things apart? [duplicate]

So, we observe that galaxies are moving apart (on average) and the further away from us they are the faster they are moving away from us. If space was expanding that would explain this. However, I ...
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When did the radiation domination end and matter domination start?

After the inflationary era the Universe became radiation dominated. The era of radiation domination is defined as the phase during which the temperature of the Universe was so high that the kinetic ...
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What wavelength was the CMB originally?

What wavelength was the light that became the CMB after factoring out redshift? Is it safe to assume that what we're seeing in the CMB was emitted at a distance of approx: ...
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Does the expansion of the universe make time pass faster?

Afaik. mass slows down time, while expansion means that the universe will be less dense in mass. Does that mean that time will go faster everywhere?
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What can I see if I hold a ruler that stretches into a distant receding galaxy? [closed]

If my clones were to form a line along that metric ruler that bridges Milky Way galaxy and a still red shifting galaxy far far away, would any of me notice anything about the scale of the ruler? Now ...
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There is what is faster than the light [duplicate]

I have read a paragraph in "Extra Dimensions in Space and Time" book https://books.google.com.au/books?id=fFSMatekilIC&pg=PA27&redir_esc=y#v=onepage&q&f=false it says that universe ...
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What makes it more likely that the universe expands instead of saying the non-expanding parts actually contract?

I have always wondered why we consider that galaxies are drifting away and not just contracting at a particle level so that locally there is no visible contraction. I mean, if gravity consumes energy, ...
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Do we really need a new physics? [closed]

I have heard many times in news that a new discovery has taken place, and we need a new physics. It has recently been announced in scientific news that the world is apparently expanding faster. And ...
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What's the definition of repulsive gravity?

After asking this question, I got confused. I always thought that repulsive gravity was a consequence of dark energy, the Nature of which is unknown. Many theories are flying around about its Nature. ...
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Is there a relationship between Black Holes and the Big Bang?

As the question mentions, is there a relationship between the Big Bang and Black Holes? It appears the Big Bang is a ‘tacked on’ theory that doesn’t align with current theories (since we don’t know or ...
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What is the current most accurate measurement of the cosmological transition redshift

Does anyone know what the current most accurate measurement of the redshift for when the Universe's expansion changed from decelerating to accelerating (with a reference)?
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Scale factor at surface of last scattering

Is there an exact theoretical scale factor associated with the CMB? More specifically, since the CMB is associated with a temperature of around 3000K (as I understand), is there an exact thermodynamic ...
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Olber's paradox with Rational Numbers [closed]

Does Olber's paradox claim that in an infinite universe (both in space and in time) light should be able to reach you from any direction? Consider the thought experiment of placing an infinite number ...
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General covariance and a running cosmological constant

A running cosmological constant $\Lambda(t)$ can always be included in the perfect fluid source tensor. By the transformation properties of tensors Einstein's field equation is still independent of ...
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Particle decoupling, universe expansion and QFT

A particle (say neutrino) decouples from the plasma in the early universe when its interaction rate $\Gamma$, with the plasma, is slower than the expansion rate $=H = \frac{\dot{a}}{a}$, i.e. $\Gamma &...
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If the universe is expanding, then the distance between the atoms must increase, which should result in decrease in density of everything [duplicate]

In the expansion of universe theory, it is already written that the scale of universe is itself expanding, which means the consequence for expansion of universe is not that atoms are growing in size, ...
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Can expanding space stretch the wavelength of GWs? [duplicate]

I have read this question: Redshifting of Light and the expansion of the universe Now analogously, we could talk about GWs traveling in the empty voids of space, where the expansion of space is ...
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Where is the work done of adiabatically expanding universe going into?

According to this Wikipedia article, the universe is expanding adiabatically. However, any system that is expanding adiabatically must be losing energy $\left(\mathrm{d}E = -p\,\mathrm{d}V\right) .$ ...
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How does cosmological principle imply an expanding space?

My understanding of cosmological principle is this: Space looks same in all directions and is homogeneous. But this principle was under heading of Observational Evidence for expansion of space in this ...
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Friedmann vacuum solution linear rather than exponential?

Does the Friedmann vacuum equation have a linear solution rather than an exponential one? Using natural units one can write Friedmann's equation for the vacuum as $$ \begin{eqnarray} \left(\frac{\dot ...
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How much did the universe expand since recombination time? [closed]

Assuming adiabatic expansion, and a temperature at recombination time of $2700^{\circ} C$, how much has the universe expanded if the temperature today is $2.7K$? My idea is to use $\frac{dQ}{dt}=A\...
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SNIa lightcurves

How do you explain the fact that in SNIa lightcurves the rate of reception of photons is less than the rate of emission by a factor of 1+z?
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Friedmann Expansion

I have a strange question that I can visualize but not find anything to really answer my query. The Newtonian law states that something in motion tends to remain in motion. We all know about ...
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Is space expanding?

Is the observed stretching of the SNIa lightcurves, ie the delaying of the emitted photons at reception, evidence that space is expanding or should this be explained otherwise and how?
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Does the expansion of space cause light to lose energy, or is it the other way around? [closed]

I am just starting Astronomy and have enjoyed reading about it. The expansion of space, the determination of the Hubble constant by looking at red-shifts, e.t.c. and it has made me wonder, is light ...
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What's going on with the Hubble constant?

I'm doing a small project on LIGO for a third-year project at University. I want to write something about how LIGO can be used to measure $H_0$, which would be useful because the current state of the ...
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As space expands does the density of matter remain same or increase? [duplicate]

I learned that space is always expanding. Even between sun and earth, it's just gravity is stronger. Imagine space as graph paper and there is a square object that fills it. Space expands, 1x1 cell ...
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How to calculate the derivative of scale factor as a function of conformal time from the solution of Friedmann equation

For the flat geometry of lamda CDM model, the solution for Friedmann equation is $$ a(t) = \left\{ \frac{Ω_{m,0}}{Ω_{Λ,0}} \sinh^2 \left[\frac{3}{2} \sqrt{Ω_{Λ,0}} H_0(t - t_0)\right] \right\}^{1/3},...
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Could a large amount of antimatter accelerate the universe instead of dark energy? [closed]

It is not known whether antimatter is repulsed by or attracted to matter. For the purpose of this question, I assume that antimatter is repulsive to regular matter (otherwise this question makes no ...
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Nature of motion between comoving observers; What is the common time that they agree on?

This is a set of follow-up questions regarding this post. The following four queries are very closely related and needed to asked at the same place. Question 1 Is it really possible to regard the ...
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1answer
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Solution of Friedmann equation in terms of conformal time

We know that for the flat geometry of lamda CDM model, the solution for Friedmann equation is $$ a(t) = \left\{ \frac{Ω_{m,0}}{Ω_{Λ,0}} \sinh^2 \left[\frac{3}{2} \sqrt{Ω_{Λ,0}} H_0(t - t_0)\right] \...
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3answers
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Is there a contradiction between isotropy and the Big Bang? [duplicate]

Disclaimer: I'm not asking whether the Big Bang happened at a point. I'm asking whether the fact that the universe is isotropic and that the Big Bang happened contradict each other. To be honest I am ...
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How to relate CMB acoustic peak to BAO in galaxy surveys to better than a factor 2?

Question for a friendly cosmologist. Let's define the sound horizon as $s = \int_0^{t_{ls}} c_s (1+z) dt$ where $c_s$ is speed of sound in primordial plasma (roughly $c/\sqrt{3}$ but you can include ...
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1answer
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Experimental status of the Big Bang theory

Take the most successful version of the Big Bang theory I'm aware that the Big Bang theory have unanswered questions related to the very first moments of the Big Bang. I also know that for our ...
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Observable universe radius for distant observers

The radius of the observable universe is about 46 Gly. Is that figure true for all current observers in our universe? Is it true if the universe is finite or infinite, flat or curved?
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Volume expansion rate

In the paper Backreaction in late-time cosmology by Thomas Buchert and Syksy Rasanen, Annual Review of Nuclear and Particle Science 62 (2012) 57-79, in eq .2.2 the covariant divergence: $$\nabla_\...