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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Cosmology: Can we compute the curve of scale factor $a(t)$ from Angular and/or Matter power spectrum?

I have generated matter power spectra (with CAMB and CLASS codes) which are in agreement: and also angular power spectra: I would like to be able to infer from these spectra the curve of scale ...
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Brans-Dicke's model: how to shift to the left the curve of scale factor to get a standard age for universe?

Using a code that implements the Brans-Dicke's model with all the modified equations of field. By doing MCMC, I have found the following estimations: $\Omega_m=0.240303,\\ \Omega_{de}=0.761206,\\ \...
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Why is the Hubble law so accurate at scales smaller than galactic voids?

It's possible to derive the Hubble law: $$v = H_0 d$$ from the FRW metric by differentiation. Experimentally the Hubble law appears to hold for relatively small distances, say 20 Mpc and smaller. ...
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Is this right that the fate of space expansion depends on matter density inside the universe?

Is it right think that a right amount of matter density inside the universe could eventually stop the expansion of the universe or the expansion of space is something intrinsic only to space so the ...
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How (if) is the metric of the quantum vacuum different from the metric of the classical vacuum?

The classical vacuum, with no matter or energy in it, has a flat metric. Meanwhile we know that the classical vacuum is a chimera. There are lots of things going on, eventhough its called virtual. ...
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Complex scale factor for a model in cosmology [closed]

I was calculating the scale factor versus the time but I just realized that a scale factor is a complex number in my model. What is the mean complex number for the scale factor?
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Comoving distance vs physical distance

In a recent post in Medium.com Ethan Siegel wrote the following: "... as you look farther away, objects appear smaller until a critical point: a minimum size that objects will appear in our ...
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Conformal Cyclic Cosmology

In this conformal cyclic cosmology, where did the very first eon come from? How did the very first eon start with possible lowest entropy?
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Is the gravitational tidal force equivalent to expanding space?

If you fall towards a black hole, the particles in front and the back of you, in the direction of the center, are accelerating away from you. So, seen from a freely frame, can we say that space is ...
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Is there a consensus among cosmologists about the big freeze or any “fate of the universe” scenario?

Any insight into the matter would be helpful.
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What would happen if the two mouths of a wormhole are receding away from each other faster than light?

If wormholes could form naturally, and point A and point B were in galaxies receding from each other faster than light, what would happen to something going from one end to the other? Physicist Kip ...
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Cosmological expansion around ropes between galaxies

The FLRW cosmology is a fair approximation of cosmological expansion for space between galaxies and clusters, however within galaxies themselves, is known to not hold, and in fact observational ...
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Will a light come back within finite years?

In this answer Javier said Imagine the universe was the inside of a ball. We're 3D now, so no one is hiding any dimensions. This ball has a border, except it's not really a border. You should think ...
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What happens when the universe runs out of fuel?

After some X billion years, one would think the stars in the entire universe will run out of hydrogen. What would happen next? Is there any way to get hydrogen out of heavy metals (extreme fission)? ...
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The space of the universe is expanding. Is there any corresponding universal phenomenon for time (contraction/dilation)? [duplicate]

Just like how electric and magnetic fields are complimentary, space and time are complimentary as they share the same "fabric". We observe that space in the universe is expanding at about a ...
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Is redshift an accurate method for measuring distances of distant galaxies and quasars?

I'm currently in 11th grade working on a science project about measuring distances using redshifts. I understand how the wavelength of a light wave increases when an object moves away from the point ...
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With how many Newtons of force is the universe expanding?

The universe is expanding at an accelerating rate. They key word here is "accelerating". Meaning that there is an equivalent "force" (in Newtons) that would cause that same ...
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In an expanding universe, can two people communicating to each other about their cosmological horizons get around their horizon limit?

I want to pose a preamble question that I will answer first to build up to the main scenario. Then I will pose the main question. The main question concerns the special case of an expanding universe ...
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How is it that we are still able to talk about the speeds of other galaxies, distances and times across the universe, etc despite GR?

Science articles often say that Andromeda galaxy is approaching us with speeds such and such. Sometimes we say that the universe is expanding away faster than light. Sometimes we talk about things ...
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What is there at a point the universe hasn't expanded past yet? [duplicate]

(Please don't mark as a duplicate) If the universe is constantly expanding that means that there is a point the univese hasn't expanded past, with that what would be past that point? This isn't about ...
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What is at the edge of space? [duplicate]

if the universe is constantly expanding that means that there is an end where the universe hasn't expanded yet if you could be at that before it was to expand more, or go past/into that edge, what ...
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Friedmann equation solution and age of the universe in terms of $H_0$ and $Ω_{(r,0)}$ for an open/closed universe [duplicate]

I cam currently doing a problem involving Friedmann equation for $a(t)$ and solving for $t$ when $a(t)=a_0$ in order to get the age of the universe. Since the integrals are quite nasty, I want to make ...
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Deriving the age of the universe

I am trying to work out the solution to exercise 8.4 from An Introduction to Modern Cosmology by Andrew Liddle. I could derive the Friedmann equation as below, $$\dot{a}^2 = H_0^2 \left[\Omega_0a^{-1} ...
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Space-time continuum expansion

I still don't understand how the expansion of the universe works. If the universe is made up of an infinite number of points that make up space-time, then how can space expand or stretch. Common sense ...
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Why does an Alcubierre drive, travelling FTL, violate causality, if the universe expanding FTL doesn't?

An Alcubierre drive seems to be plausible as a means to travel faster than light, because it doesn't move the object itself, but the space around it. it's said that matter and information can't move ...
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How is the divergence of a unit timelike vector related to expanding space?

Just for fun I'm trying to understand spacetime with an expanding universe from a 3+1 point of view (yes, similarly to the ADM formalism). If I consider our spatial 3-surface $\Sigma$ as embedded ...
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Why is the age of the universe $=D/v$, despite that $v$ is not constant with time?

I am watching a series of lectures by the Noble prize laureate Brian Schmidt and Paul Francis and in this episode (at 4:20) they make the simple assumption that a galaxy receding from us due to the ...
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Does the expression "$𝑑𝑠^2$..." mean the same thing as "$\Delta 𝑠^2$... "?

I reviewed this question but sometimes I'm unsure about delta versus differential notation. Does the expression "$ds^2=-c^2dt^2+a^2(t)[dr^2 + S_k^2(r)d\Omega^2 ]$" mean the same thing as &...
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What if cosmological inflation never stopped?

If inflantionary theory is correct, the universe ceased to expand after just $10^{−32}$ seconds. Imagine a scenario where cosmic inflation never actually stopped. At the extremely accelerated ...
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Can dark energy expand the distance between the earth and the sun?

I remember that the expansion is 0. Is it exactly 0 or just really close?
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Can space only be infinite? [closed]

I have read before that if you could just go fast enough, as a thought experiment, and you move in a straight line, in any direction, that you eventually might reach the spot from which you started. I ...
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If the universe expands uniformly in all directions, wouldn't that make the basis for an universal and absolute "now" and reference frame? [duplicate]

Let's consider a universe with constant expansion for simplicity's sake. In such a universe, the Hubble Parameter drops to half its value after double the time. If it happens to be 70 km/s/Mpc today, ...
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Adiabatic expansion of the universe

I was wondering why we consider the universe to expand adiabatically. If the universe is a closed system shouldn't be the energy conserved? Does the adiabatic expansion change the total internal ...
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Constant speed of light violates accelerating expansion of universe?

My question regards the following: One of the most fundamental principles of Einstein's GR is that all free bodies move through spacetime with constant velocity $c=1$. However, in 1998 Hubble showed ...
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Measuring the Hubble Constant

Attempts to measure the expansion of the universe have come in various forms. The recent Cosmology Crisis (https://www.space.com/why-is-there-a-cosmology-crisis) has me pondering the expansion rate ...
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What is the “conspiratorial anisotropy” if we would see no star redshifts in one direction of the sky? [closed]

Suppose we see no redshift of stars in one direction only (or better, in directions on a small patch around it). And suppose in that direction (on a patch of sky around it) we see a lot more stars ...
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Distance Modulus as Function of Redshift

This question comes from Ryden's "Introduction to Cosmology," 2nd Ed. Problem 6.4 I'm asked to verify the following equation (which I believe just means to do the algebra and get into this ...
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Can the age of the universe be much bigger than 13.8 billion

If observable universe is only a small fraction of the existing universe, does it imply that the age of the universe is much more than 13.8 billion years or the expansion of the universe is much ...
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Ratio of a given co-moving length scale to the horizon

How does the quantity $aH = \dot{a}$ measures the ratio of a given comoving length scale to the horizon (during inflation say)?
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FLRW metric, universe expansion, and the energy-momentum relationship

This is a follow-up to a previous question of mine. I am getting myself confused by some basic things in cosmology, so I hope whoever reading this is patient. The Euclidean FLRW metric is given by $$ ...
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The average density inside the particle horizon (observable universe) $ρ(t)$?

Does anyone know the average density of our universe and the particle horizon function as a function of time? The average density inside the particle horizon(observable universe) : $ρ(t) $? Radius ...
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$Ω = ρ/ρ[c]$, so although near to 1, for an accelerating expansion $Ω$ must be below 1. What's its value?

$Ω$ is taken to have different components - ordinary matter, dark matter, dark energy. But because it is expressed in relation to the critical density for attractive gravity, it seems that omega is ...
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Validity of Friedmann's equation

Recently, a friend gives me the Friedmann's equations under the following form: \begin{gathered} \left(\frac{\dot{a}}{a}\right)^{2}=\frac{8 \pi G}{3 c^{4}} \rho+\frac{\Lambda}{3}-\frac{k}{3 a^{2}}\...
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Observed redshift changes at a rate

I'm having a bit of trouble showing that in a flat, single component universe, the redshift changes at a rate modeled by $$\frac{dz}{dt_0}=H_0(1+z) - H_0(1+z)^{3(1+w)/2}$$ My current steps involve the ...
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Redshift Due to Expanding Space (not increasing distance) [closed]

Please NOTE: "For example, a question that proposes a new concept or paradigm, but asks for evaluation of that concept within the framework of current (mainstream) physics is OK." And: "...
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Does general relativity prohibit spacetime expanding in a higher dimensional space?

Are there theoretical considerations that prohibit the 4-dimensional spacetime as described by general relativity to be situated in a higher dimensional spacetime? If not, could that explain why the ...
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Dark energys energy density value

What mathematical property does dark energy have that causes the effect of an expansion? I have read that the energy density of dark energy is time invariant, which is the main cause of an accelerated ...
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Could we estimate the total energy of the universe?

So I know that we do not know the sum of all energy in the universe, but why can we not just estimate with the following logic? (That I assume has some fatal flaw preventing anyone from guessing the ...
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Could our universe still work if either one of $c$, $h$, $q$, $G$, permeativity, permeability or $H_o$ were slightly different? [closed]

Could our universe still work if either one of c, h, G, q, permeativity, permeability or Ho were slightly different? Also all of them are thought of as being constants for the whole history of the ...
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Relationship between expanding space and dark energy [duplicate]

The excerpt below has been taken from a pop-science article. Dark energy is thought to be different, though. Rather than being a type of particle, it appears to behave as though it were a type of ...
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