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 are things far away moving away faster?

This is a really dumb question. But if things farther away are red shifted more, that literally means that the further away in time, things were moving away faster, but things closer to us arent ...
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Korean SAT question error: Hubble law

I brought some of the questions from the Korean SAT. In the context of Hubble's law, galaxies A, B, and C appear like pictures. (The $y$-axis is the recessional speed, and the $x$-axis is the ...
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Where does the energy from expanded light go? [duplicate]

The universe is expanding and light traveling through it also expands. Light with longer wavelengths have less energy so light is losing energy as the universe expands. Where does this energy go? Or ...
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Is there a GR explanation for cosmological coupling causing mass increase as the universe expands?

A paper in November 2021 considered the hypothesis that things with mass, including black holes, may be coupled to cosmological expansion so that they slowly gain mass as the universe expands, similar ...
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Absence of matter stretches the fabric of space?

If matter attracts matter, because it curves the space, could it be that the absence of matter stretches the space, and that's why the universe expansion is accelerating? I mean that could be ...
Jose Renato's user avatar
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What is the relationship b/w mass’s effect on the curvature of space vs the expansion of space?

As is well known, General relativity explains that mass and energy bend the curvature of spacetime. Mass energy of different amounts lead to different space time curvatures. As is also well known, the ...
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Big Bang and relativity [duplicate]

Wasn't the big bang's explosion itself faster than speed of light? how does this not violate relativity, I had read an explanation earlier but it wasn't sufficient, can someone explain to me how the ...
just some guy on the internet's user avatar
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Where did the CMB come from? What is due to the matter/antimatter annihilation? Or perhaps the radiation released when the electrons decoupled?

Where did the CMB originate from? I get that at the beginning of the universe, by the Big Bang theory temperatures and pressures were too high for matter to exist, and even if it did, it would just ...
physicsphil's user avatar
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Local effects of cosmic expansion

Suppose we place two galaxies 100 Mpc apart, with zero initial velocity with respect to each other. In other words, they are static with respect to each other initially, with a negligible ...
user157588's user avatar
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4 answers
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Did the universe need the presence of matter and radiation to start expanding?

I have read this question: Hence it is not possible that photons generated by stars is contributing to dark energy. Could photons generated from the many trillions of stars be some how contributing ...
Árpád Szendrei's user avatar
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7 answers
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Is there any "Cosmological Principle"?

The term "Cosmological Principle" is widely invoked in cosmology discussions. I think the basic idea here is that one might guess that the Milky Way is not at a special location in the ...
Andrew Steane's user avatar
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Do more distant galaxies move faster with respect to the cosmic background radiation rest frame?

Aparently our galaxy is moving approximately 1/500th lightspeed with respect to cosmic background radiation. While this is certainly fast, it stands to reason that it could had been a lot faster, so ...
Derek Seabrooke's user avatar
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1 answer
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Is cosmic background radiation consistent with the Cosmological Principle? [duplicate]

Is the observation of cosmic background radiation really consistent with the cosmological principle? It implies that there is a "special" rest frame of motion with respect to the big bang. ...
Derek Seabrooke's user avatar
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2 answers
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If Andromeda galaxy is 2.5 million light years away, shouldn't the light we see be younger than 2.5 million years?

I was wondering about this. I've always heard that (for example) Andromeda being 2.5 million light years away means that what we see is from 2.5 million years ago. However, taking into account the ...
user5715636's user avatar
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1 answer
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Dilaton field causes universe expansion?

In string theory low-energy $n$-dimensional gravity is described by an action of the following form: $$S^{(n)}=\frac{1}{2\kappa^{(n)}}\int d^nx\sqrt{-G}e^{-2\Phi}\Big(\mathcal{R}+4\partial_\mu\Phi\...
John Eastmond's user avatar
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2 answers
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Why do matter and radiation densities scale differently in expanding universe, when all matter is energy and all energy is matter? [duplicate]

I sometimes see the first Friedmann equation expressed with different terms for the density of "matter" and the density of "radiation". It is said that with an increase in scale ...
user102008's user avatar
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Does GR explain cosmological time dilation?

There is plenty of evidence for a cosmological time dilation effect. For instance a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift $z=1$ (see Ned ...
John Eastmond's user avatar
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Why do some diagrams of the particle horizon, observable universe etc show the past light cone as hitting ~20glyr out from us at time zero?

This question is mainly in reference to this question: Is the observable universe equivalent to 'our' light cone?, and the answer, which is great. But what I can't wrap my head around is why ...
Jack Mace's user avatar
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What kind of matter's energy density scales as the inverse of the scale factor

We know that radiation energy density scales as $a^{-4}$ with EoS parameter ($w=\frac{1}{3}$), matter as $a^{-3}$ with ($w=0$), curvature as $a^{-2}$ with ($w=-\frac{1}{3}$). Then which kind of matter ...
Faber Bosch's user avatar
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A fire at the end of inflation?

In their article at bigthink.com "Surprise: the Big Bang isn't the beginning of the universe anymore", the writer articulated an alternative theory that suggests inflation didn't begin at a ...
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Does Time Dilation apply to me when I'm outside the Hubble sphere?

Imagine that I'm at a fixed point on space outside the Hubble sphere. As the space outside the Hubble sphere expands faster than the people on Earth, I too should be moving away from you guys at the ...
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How the universe is expanding? [duplicate]

If universe is expanding with the velocity of light then why the distance between earth and sun is constant? Also why the distance from one Galaxy to another remains constant? And so on...
Large Hadron's user avatar
2 votes
1 answer
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Do particles at rest gain energy in an expanding universe?

It is commonly understood that as the Universe expands with scale factor $a$ the energy of a photon drops like $1/a$ whereas the energy of a particle at rest is constant. In the analysis below I ...
John Eastmond's user avatar
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How fast is the edge of the observable universe expanding from the Earth?

If the universe is expanding at 73 kilometers per second per megaparsec, what is the fastest expansion speed that we can observe from Earth? I'm assuming that's the edge of the observable universe ...
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Black Holes, Gravity, and the Creation [duplicate]

As I suppose we all accept, the universe started as an unintuitive singularity—either, "Let there be light," or "The Big Bang". Something from nothing expanded and at luminal ...
ClancyJohn's user avatar
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1 answer
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Does the expansion of the Universe cause internal stresses in solids (and other substances with similar internal resistance forces)? [duplicate]

Is my understanding correct? Weren't it for the internal resistance forces between particles in a solid object, the expansion of the Universe would continuously increase the distances between all of ...
jvf's user avatar
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What are the biggest shortcomings of the Big Bang theory?

Big bang theory is the most accepted theory by the physicists. It explains about the origin of the universe but what are the loopholes of this theory? Is the loophole about time or space or the origin?...
Prakhar Soni's user avatar
3 votes
1 answer
722 views

Is the size of far away galaxies smaller or larger than expected?

When items are further away, they look smaller. If we know the size of something nearby, then we can calculate the size of something a known distance far away. So if there is a galaxy whose size (...
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2 votes
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What would the size of the observable universe be if you traveled 3/4 the speed of light?

I have a conceptual mess in my head and would like to clean it up. From the perspective of Earth, we can measure the observable universe diameter at current at 93BLY. I understand this and why/how ...
Brian Woodward's user avatar
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3 answers
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Does an expanding universe cool down?

I want to know if and why the temperature of an expanding universe decreases with time. Universe setup. My universe is modeled as the manifold $]0,\infty[\times\mathbb T^3$, where $$\mathbb T^3 = (\...
Maximilian Janisch's user avatar
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What was the last point in time when you would have been able to see yourself via light wrapping around the universe?

To put it another way, if you are floating in space and light reflects off of your back and travels outwards through the entire universe, and reaches your eyes on the other end, how far back in the ...
Ethan's user avatar
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7 votes
1 answer
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Energy extraction from universe expansion? [duplicate]

I recently learned that energy is not conserved according to general relativity. Inspired, I went looking for a way to harness this interesting feature of the theory and came across an idea that ...
Cody Cox's user avatar
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1 answer
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Minkowski Space and the Expansion of the Universe

I was watching a video about Minkowski space. It stated that no past event, outside of the light cone in space can be a cause of an action inside the light cone. Is this due to the expansion of the ...
Rick's user avatar
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Does expansion of space over time assume a particular space/time dichotomy?

Regarding the expansion of the Universe, Wikipedia states: The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe ...
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Is it possible that dark energy could be non-uniformly distributed?

Is it even possible that dark energy could be non-uniformly distributed? If it were the case at some points where creation is faster would some objects like for example stars move apart with a rate ...
Janko Bradvica's user avatar
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1 answer
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Is red-shifting the only proof for the expansion of the universe?

Are there other proofs for the expansion of the universe other than red-shifting? If yes, what are those? Thank you.
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Is there a specific reason that the universe is precluded from expanding into anything? Or is it simply not required in the mathematics? [closed]

We know that the universe doesn't need to expand into anything to support current theories, but is there a reason that it is specifically precluded from doing so? In other words, is there any evidence ...
Jack's user avatar
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16 votes
1 answer
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Galaxy getting closer faster than light

In cosmology, we speak of galaxies racing away from us due to the expansion of the universe. The light from these galaxies appears red-shifted. If the galaxies are far enough, then the space between ...
Alon Navon's user avatar
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1 answer
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Space expansion explainable by a 4D universe?

Could our universe be finite, but in 4D? Basically that the universe that we experience is a 3D slice of a 4D body that moves through our 3D "plane". The "big bang" then occured ...
Gevatter Gaul's user avatar
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1 answer
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What is the 'effective number of neutrino species'? And how does that, rather than the total number of them, affect the universe?

As stated above... How can the the physics of early cosmology (articles about which are where I usually come across mentions of such) be affected by an 'effective' number of neutrino types, rather ...
Kurt Hikes's user avatar
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Explain Hubble's law using proper distance and comoving distance

As far as I can understand, Hubble's law has the form of V(to)=HoDo at a close distance. The meaning of this equation is as follows. For example, when z=0.01, the emitted photons arrive at us and tell ...
teacher's user avatar
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Suppose you are told that the linear size of everything in the universe has been doubled overnight. Can you test this statement? [duplicate]

Suppose you are told that the linear size of everything in the universe has been doubled overnight. Can you test this statement by measuring sizes with a meter stick? Can you test it by the fact that ...
Sheldon Cooper's user avatar
1 vote
0 answers
104 views

Affine space in classical mechanics and it's applicability in general relativity

In the first chapter of Arnold book of Classical Mechanics while giving Galilean structure of spacetime we're introduced to affine space. As already mentioned in answers to this question this is done &...
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Temporal Variation of Cosmic Microwave Background Radiation

For some time now I am wondering how fast the Cosmic Microwave Background (CMB) Radiation varies with time, in particular, how fast the primary CMB anisotropies are varying. These anisotropies were ...
emacs drives me nuts's user avatar
1 vote
2 answers
190 views

Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe than the 'present' closer part of the universe?

Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe?If the universe is expanding, the galaxies should have been much closer in the past so should we see a larger ...
Janko Bradvica's user avatar
2 votes
1 answer
573 views

Is Hubble graph straight line or curve?

According to this paper, the graph explaining Hubble's law is expressed in two cases. The first is when the hubble parameter is constant over time, and the second is when it changes over time. My ...
teacher's user avatar
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18 votes
2 answers
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How long ago was the Universe small enough for interstellar travel?

Currently, even the nearest stars are lightyears away, and impossible to reach in our lifetimes. If space is always expanding, and was once infinitely smaller, then at what point in the past was space ...
Ben Warner's user avatar
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1 answer
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How could the concept of finite unbounded universe be compatible with the fact that there are 3 dimensions? Is the example of the balloon a bad one?

In cosmology, the current understanding is that the universe is finite while being unbounded. To explain that, one often uses the example of the surface of a balloon: if we are on the surface of the ...
Mathieu Krisztian's user avatar
3 votes
0 answers
204 views

Hubble's law for approaching, rather than receding, galaxies

Hubble's law states that, the recessional velocity $v_r$ of a distant object, relative to an observer, is $$ v_r = H_0D + v_{pec} $$ where $v_{pec}$ is the peculiar velocity, $H_0$ is the Hubble ...
wzkchem5's user avatar
1 vote
1 answer
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Does the model of how a ball falling through Flatland would appear to a Flatlander have any implications for theories about the Big Bang?

It struck me that the Flatland example has some similarities (for me as a non-physicist) to the way the Big Bang is described. i.e not an explosion, but an expansion that is accelerating. My limited ...
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