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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Transparency of hydrogen to different electromagnetic wavelengths in the early universe

I can somehow grasp the early universe being compared as a sphere surface filled densly with plasma like a spherical chessboard all filled with chess pieces.So as that surface streched the distances ...
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Is energy conserved during the expansion of the universe? [duplicate]

Consider the following thought experiment. Suppose we initialize a parallel plate capacitor with the plates of area $A$ separated by a distance $d$ in empty space. The energy in the capacitor is ...
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How to explain apparent acceleration due to the expansion of the universe and inertial reference frames?

In Newtonian physics, object A observes the acceleration of object B precisely when A observes a net force acting on B. How should we understand perceived acceleration under the observed expansion of ...
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Friedmann Metric and congruence

I am told to show that the congruence in the Friedmann Metric is geodesic. I am really confused how to solve it, because i am not sure how to "find the congruence". I thought that i could ...
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Absence of global simultaneity, block universe and estimated time since big bang [closed]

It is said that the big bang happened about 13.8 billion years ago. It is computed by playing backwards the observed movement of galaxies all over the universe. If there is no global notion of ...
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Expansion of universe effects on mass

It has been noted that the metal alloy model previously used to characterize the value of 1 kilogram has been steadily increasing in mass due to unknown causes. Could this be a local effect of ...
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Is it theoretically possible to determine the age of the universe to a high precision?

My question is a bit abstract, but I'm wondering if it makes sense to expect/hope that one day we will be able to measure the age of the universe (in Earth's frame of reference) much more precisely, ...
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On the assumption that the universe is not expanding, but things are shrinking [duplicate]

Although the title gets the gist across, clearly, this is going to run into a lot of contradictions if I don't explain the idea as it resides in my head. Bear with me in the following analogy of a ...
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Are galaxies moving away from each other at a constant speed?

Im trying to understand what Hubble's law $$v = H_0 d$$ means. The thing is I keep getting two different explanations. Galaxies are moving at a constant speed, the ones further away are so because ...
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Proof of an accelerated expansion of the universe [duplicate]

Are there any more "proofs" of an accelerated expansion of the universe other than: $1:$ that galaxies that are farther away from us are also more redshifted. $2:$ using "Baryon ...
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What is the exact form of Hubble's Law in a flat FRW universe?

I would like to know the exact form of Hubble's law for the luminosity distance $d_L$, where $d_LH_0=f(z)$ in a flat FRW universe with energy density dominated by a component satisfying $\rho \sim a^{-...
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If spacetime is expanding, shouldn't lengths expand at an equal rate? [duplicate]

I frequently hear that galaxies are moving apart (in some instances, faster than the speed of light) because the fabric of spacetime is expanding. The common analogy is two people standing on the ...
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Why do the TT, TE and EE power spectra in the CMB lead to different values for the Hubble constant?

Table 2 of Planck 2018 results Why are these three values different? In particular, why is the EE value so much closer to the local universe value, and have so much higher error?
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The maximum distance of gravitational attraction

I'd like to better understand how gravity functions at extreme distances. Here's what I'm struggling with: In Newtonian mechanics, every mass in the universe attracts every other mass inversely ...
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Did the Big Bang cause an outward push of gravity?

There is a theory that the big bang’s blast caused an outward push, a kind of reverse gravity if you will, of our universe and everything within it. My question is how could this have happened? If it ...
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How does the Planck satellite's detailed map of the CMB lead to a value of the Hubble constant, $H_0$?

Cosmologists have put great faith, it seems, on the cosmological model that led to a value (of 67) of the Hubble 'constant', after carefully peering at Planck's map of the cosmic microwave background.....
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What happens to the energy of photon red shifted by expansion of universe?

There are many similar questions but the answers don't satisfy me. I mean it as asked in the title. Some answers given before give the explanation based on gravitational potential energy but consider ...
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Negative mass density in cosmology?

I came across a problem in Chapter 6 of An Introduction to Modern Cosmology by Andrew Liddle that reads: Identify a sufficient and necessary condition that must be satisfied by the equation of state ...
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Directional dependence of scale factor $a(t)$

Why does the scale factor $a(t)$ in FLRW metric has only temporal dependence but not directional? If $a(t)$ had a directional dependence, I think it would violate the cosmological principle since The ...
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While space itself expands, why does it seem to expand more between the stars than between me and my desk? [duplicate]

this is my first post in this community and I am not entirely sure if it is appropriate. I am not an educated physicist, just an ethusiastic person trying to understand the universe. So I apologise if ...
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Why the FLRW metric was chosen with constant $g_{00}$ time component?

The FLRW metric is used to describe Universe expansion. Why for this purpose time component of the metric is chosen to be constant? Even other metrics which describe inhomogeneous Universe expansion ...
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What are the current theoretical solutions to the Hubble Tension?

As it has been known in the last few years, there is a discrepancy in the measured values of the Hubble constant, depending on whether the method addresses early or late times of the cosmological ...
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Motion vs space expansion

Let us consider two particles A and B in empty space, and let us assume that I am an observer attached to particle A. Also let us assume that, according to my measurements, I see that the distance ...
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Does time expand?

If the time component of spacetime does not "expand" in the sense that space expands, doesn't that imply that time must already exist in its entirety, past and future? I believe I have seen ...
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Where does $H = \frac{M_0}{T^2}$ come from?

In this case: $M_0 = \frac{M_{Pl}}{1.66 g_*^{\frac{1}{2}}}$ and $H$ is the Hubble constant (which is not always constant depending on he constitution of the Universe). where $M_{Pl}$ is Planck's mass ...
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What is $a(t)$ in FRLW metric?

The Friedmann-Lemaitre-Robertson-Walker metric (FLRW metric) is described as: $$ds^2 = dt^2 - a^2(t) (\frac{d \bar{r}^2}{1-K\bar{r}^2} + \bar{r}^2 d\Omega^2)$$ What does $a(t)$ represent? I know that ...
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How to tell the difference between redshift by motion of stellar objects and the expansion of space? [duplicate]

By observing how much the say hydrogen spectrum line was shifted we can tell whether the source is moving closer or further away, but then clearly cosmological redshift do not work this way so how can ...
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How do you know that objects are accelerating from each other in space?

I know that Hubble redshift comes from the relation between the distances of objects and the Hubble parameter: $$H_0 = \frac{\dot{R}}{R}.$$ But how we differentiate between redshifts effects from the ...
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How would the expansion of space affect a very long rope? Why would the effect not violate the law of conservation of energy?

This question isn't fundamentally new, but I haven't found one that elicits a clear answer on the conservation of energy problem. Another question asks: "How Does Hubble's Expansion Affect Two ...
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As space expands, should the Planck length too?

The Planck length is usually defined as the scale at which QM effects become dominant. This is what I am referring to. I found one question, but that one has no good answers. How does universal ...
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What is the combined explosive force of the Black Hole Era?

It is theorised that in the distant future all matter will gravitationally attracted into Black Holes. The universe will then be seemingly still for a long time until the Black Holes have all ...
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Deriving an equation from the energy-momentum tensor in general relativity

I have a mathy question regarding the Friedmann-Lemaître equations (FLE) in standard cosmology. For reference the FLE are as follows $$\frac{3}{R^2}\Big(k + \frac{\dot{R}^2}{c^2}\Big) = \frac{8\pi G}{...
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Euler equation in comoving coordinate

I have the following cosmology exercise : I found many hints on internet : first source, second source But I still can't figure it out. If we is start from \begin{equation} \frac{\partial \vec{v}}{\...
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If the universe is expanding at an accelerated rate, was it stationary at some point in time and contracting before that?

If the universe is expanding at an accelerated rate it would mean that the more we go in the past, the slower the speed of expansion must have been. Provided the acceleration was always non-zero and ...
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Conformal Cosmology Confusion

When we look into the far future of the universe, it seems possible that it might reach a state where photons are basically all that remains. Roger Penrose states that the universe would ‘forget’ its ...
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Subhorizon vs superhorizon

In this review (page 18 top) it is argued that superhorizon modes are defined as $k_{\rm phys}<H(t)$, while subhorizon modes are defined as $k_{\rm phys}>H(t)$. What is the physical ...
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Are we seen or not by an observer from a galaxy farther away than the age of our solar system?

I looked at this question. There's one answer that alludes to what I'm asking, but I don't find it satisfactory, because to me the following is still a paradox: The most distant galaxies we have ever ...
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How is Hubble constant measured by the method of observing BAO?

How is Hubble constant measured by the method of observing BAO?By just reading some articles it seems to me a more complex method than the astronomic scale.Can this method be explained in few words to ...
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How does universe expand when cosmological constant is zero?

From what I learned, Einstein believed in a static universe but from his general relativity equations universe must collapse under gravity. Hence Einstein adjusted this gravity with cosmological ...
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Is the Big Bang occurring or did it already occur (a question about time) [duplicate]

I understand how time (space/time) is perceived according to the theories of relativity, but I still don’t understand: Did the Big Bang occur 13.8 billion years ago or is it occurring relative to us ...
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Why energy cannot be created nor destroyed if the Universe is still growing in size? [duplicate]

Why energy cannot be created nor destroyed if the Universe is still growing in size?There is creation of new space and infact this space is so energetic that causes 'Hubble flow' of matter...
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What equation precisely defines the epoch of recombination? [duplicate]

I understand that a very rough first estimate would be to assume this happens when the mean photon energy drops below the value required to photoionise hydrogen, namely when \begin{equation} \label{...
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Explaining accelerating spacetime expansion?

I should begin by noting that I am NOT a physicist, I study differential geometry and so my physics background is extremely limited. That being said I was reading a textbook on general relativity and ...
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Matter moved faster than photons at big bang? [duplicate]

The observable universe is 90 billion light years across. But the age of universe is 13.8 billion years. Light would take 90 billion years to cross this distance, how could matter move that far in ...
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Will spiral galaxies collapse when expansion of the universe isolates them from the rest of the universe? [closed]

If a galaxy gets isolated when the speed of the expansion of the universe exceeds the speed of light, will it collapse? The rotation of a spiral galaxy prevents it from collapsing due to gravity. So ...
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Einstein and his so-called biggest blunder

I was reading this article, https://earthsky.org/space/this-date-in-science-edwin-hubble-and-the-expanding-universe . Please note that my understanding is very basic. You could find my main question ...
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Does the expanding space apparently slow down a very distant photon that moves towards Earth?

To make a picture of what I am asking I have to use a comparission...So let say Adam walks on a 10km long bridge and wants to reach Sara at the end of the bridge but as he walks the Sun heats the ...
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What is the jerk parameter in terms of the Density parameters and equation of state?

In cosmology the deceleration parameter defined as the $$q_0 = \frac{1}{2}\sum_i\Omega_{i,0}(1+3w_i)$$ Is there a similar expression for the jerk parameter ($j_0$)?
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Does a photon travel more than 1 billion light years in 1 billion years due to the expansion of space?

Assuming we shoot a photon out into space, it would go 1 light year after 1 year, and 1 million light years after 1 million years. Because after that the expansion of space is noticeable, would photon ...
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Seeking a plot of the energy history of the Universe

How can we plot the energy density $\rho$ as a function of the age $t$ of the universe (or temperature $T$) from the time of radiation domination to the time of cosmological constant domination with ...

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