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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Can gravity weaken due the inflation of the universe?

As I understood the gravitational force is exchanged between particles at the speed of light. If there is an increment of distance between astronomical objects is it true to think that the ...
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Since the CMB is the oldest thing we can see, how do we know FOR SURE what happened BEFORE the CMB?

I keep wondering about how can we be so sure about what happened BEFORE the CMB, given that it is the oldest thing we can actually see. It seems like we are very confident about what really happened ...
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Is there any method to know the real age of the universe? [duplicate]

Well, I was wondering about the real age of our universe, I found that it's estimated to be $13.8\times 10^9$ years. Is it an approximation, or laws behind this age?
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Scale factor in conformal time

I'm having trouble getting the scale factor for conformal time. In de Sitter we've $$a(t)=\sinh(H_It), -\infty <t <\infty$$ and the relation: $dt=ad\eta.$ I don't see how this relation ...
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Friedmann equations and different types of universes

I'm currently reading a book (in Danish) about the Friedmann equations and I have stumbled upon a third degree polynomial: $$ \Omega_\Lambda = \frac{4K_0^3}{27\Omega_0^2} = \frac{4(\Omega_0 + \Omega_\...
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Restart the Universe expansion? [closed]

Is there anything that could restart the universe expansion? Basically a black whole or something else entirely different? It seems to me that, once the Universe stops expanding, the human life ...
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Expansion of space or contracting of matter? [duplicate]

Can the expansion of the Universe effect be explained by contraction of the overall matter instead? That is, all material objects are in fact contracting, due to what the universal space seems (and ...
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Does light from source one mil. light years away take more than 1 mil. years to reach us?

Because of the expansion of space itself, did light from a galaxy one million light years away actually take more than one million years to reach us? Also, if we are looking at a galaxy one billion ...
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Alternative to Dark Energy

If Black Holes consume mass and re radiate it in the form of massless photons the mass of the universe is declining. Could this explain the accelerating expansion of the universe without the ...
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What happens in the distant future of the universe?

In the standard cosmological model LCDM the universe approaches the asymtotic deSitter state in the distant future. Is it correct to say that all light-sources situated on the ultimate lightcone of a ...
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Is the relativistic Doppler effect the only way to show that the universe is expanding?

Is there another way to show that the universe is expanding, rather then only relying on the relativistic Doppler effect?
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Is it possible to see the history of space if it expands faster than the speed of light? [duplicate]

I was wondering if it would be possible to see the history of space if one was to stand at the edge of the universe as it expanded faster than the speed of light. If the universe was to be expanding ...
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If energy is neither created nor destroyed, where is the extra space coming from? [duplicate]

I've been learning about many aspects of science all of my life, and have come to understand the following: Energy is neither created, nor destroyed. (Classical Physics) Space is expanding. (...
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How long would it take to get to the Hyperion proto-supercluster?

Right now, we set a ship carrying no humans going to the Hyperion supercluster (redshift is $z=2.45$, so around $11$ billion light years from Earth) at a velocity of $c/2$ ($c$ being the speed of ...
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If space expands, why does a liter of water stay a liter? [duplicate]

From observing the universe, we know that space expands. I see what that means on cosmic scales. But what does it mean on smaller scales? If I have a graduated beaker of 2 liter, and 1 liter of ...
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Galaxy is 8 times closer 13 billion years ago? [duplicate]

Based on below, How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ? I ...
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Calculate distance of galaxy 13 billion years ago [duplicate]

How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ?
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How do we know that the space metric is expanding as opposed to objects moving relative to one another?

So we observe that the Universe is expanding. We observe galaxies moving away from us and the speed/red-shift is greater for galaxies that are further away. How do we know that this observation is ...
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If everything in the universe is drifting away will We eventually be able to register things disappearing?

If everything in the universe is slowly drifting away how soon before we see signs of this?Maybe we’ve already registered things at the visible edge of the universe disappearing. If not will, it ever ...
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Accounting for expansion in $N$-Body gravitational simulations

I have an $N$-Body simulation for the evolution of the universe. It uses a hybrid Tree Particle-Mesh scheme to compute the forces. The tree code used Newtons law of gravity: $$\textbf{F}_i = -G \sum_{...
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How is Universe expansion measured with CMB

There are plenty of articles about measuring the universe with standard candles, parallax, certain types of stars, etc. CMB measurements are usually introduced into the article to bring attention to ...
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Spacetime stretching [duplicate]

When spacetime stretches, does both space and time stretch proportionally. That is, if you stretch Planck-length doesn't Planck-time also stretch proportionally? I find this to be a conundrum.
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Does Another Universe Where Time Runs Backwards Revive the Idea of a Cyclical Universe?

https://www.pbs.org/wgbh/nova/article/big-bang-may-created-mirror-universe-time-runs-backwards/ This article talks about a new theory where the big bang happened in one direction for us and the other ...
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What is the Relation between the Cosmic Microwave Background and the Hubble Sphere?

I've learnt that The cosmic microwave background (CMB) is the light emitted just after the Big Bang and which is reaching us now after being hugely red shifted by our expanding universe. Now Hubble ...
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Why is the scaling factor in the Robertson-Walker metric squared?

Not much to add beyond the title. The Robertson-Walker metric solution to the field equations has the form $$g_{\mu\nu}dx^\mu dx^\nu=-dt^2+a^2(t)\biggl(\frac{dr^2}{1-Kr^2}+r^2(d\theta^2+sin^2\theta \...
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How does the distribution of entropy in the expanding universe evolve? [duplicate]

Also, Is the entropy divided by the volume of the (observable) universe increasing with respect to time or is that number decreasing, unstable, or constant, or infinite?
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What is the evidence for the existence of a Hubble sphere beyond which galaxies recede faster than light

The title has been changed from the original title, because that is what I meant, but I misused the terminology, and probably some physics, asking about evidence for the existence of a horizon for the ...
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Age of Universe from Friedmann Equation - How to actually solve the integral?

The Friedmann equation for a flat universe can be written as $$ H(t)=\frac{\dot{a}}{a}=H_0\sqrt{\Omega_{m,0}\cdot a^{-3}+\Omega_{\Lambda,0}}=H(a) $$ To calculate the age of the universe, many books ...
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Various scales of infinite

I was reading a great answer by John Rennie earlier (and took a mental note of the question which I must have mislaid). I wanted clarification by comment so I thought I could look it up amongst John's ...
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What caused Big Bang to bang? [duplicate]

What made the big bang start in the first place? I don't see what could have caused it. What made it burst apart?
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The flaws in this explanation of why the universe is so big [closed]

We all know universe is very very big (I’m talking in general meaning of words like “we all know” and “big”) and if we ask why is it so big then it might become a little bit philosophical but what are ...
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Energy is conserved. Dark energy has a fixed value per volume. The universe is expanding. Is non-dark energy decreasing? [duplicate]

I was thinking about this yesterday. Basically, these three premises: The amount of energy in the universe is conserved. Dark energy is a fixed number of joules per cubic meter. The universe is ...
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Why are galaxies so far apart?

With the Big Bang, one would expect the distribution of mass to be somewhat uniform, but we have galaxies that have billions of stars and there are huge distances between galaxies. Why is there is so ...
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How do you know what is the fate of the universe in a modified theory of gravity?

Usually when you talk about the final stage of the universe you assume that $\Lambda CDM$ is the correct cosmological model and with that you can analyze what could happen for different values of $\...
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Why did Einstein introduce expansion in his equation and later on it was patched?

I read that in developing GR, Einstein's original equation indicated that the universe is expanding and later he quickly dismissed it by adding the cosmological constant. I kept wondering, why did he ...
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Universe expansion acceleration

[I've read, among others, other questions on this, don't believe this is a duplicate] I (think I) understand the evidence that distant galaxies are moving away from us faster than closer galaxies. ...
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Time in Universe Chronology - Duration of inflation period

When you read articles about the chronology of the Universe, you read for example that the duration of the inflation era is roughly $10^{-32}$s. We also know Universe expansion causes a redshift ...
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If Hubble parameter varies with time how much it differs for low distances (present) and deep space (past)? [duplicate]

If deep space is shown as it was in the past how can we calculate this time lagging regarding the Hubble parameter which is a variable of time?
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What was the rationale behind Stephen Hawking's “biggest mistake”?

Albert Einstein said he made his biggest blunder, while Stephen Hawking spoke of his "biggest mistake". One can read this in this interesting book review (The Nature of Space and Time by Stephen ...
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Is there a “central” cosmological inertial frame?

I know there are no preferred inertial frames from the perspactive of special relativity, and I assume the same is true from the perspective of General Relativity. However, if the same is ...
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Models of accelerated expansion

Are there models of accelerated cosmological expansion that involve a changing number of spatial dimensions? I think I've seen a version of string theory that posits a reduction to quasi-3 dimensions ...
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Hubble parameter

When we observe a distant galaxy we must see that as it is moving from us its speed regarding Earth increases according hubble constant.But as seems it is not a constant but an increasing function of ...
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Why is CMB an evidence of hotter and denser early Universe?

In his book Gravitation and Cosmology, Steven Weinberg says that the Cosmic Microwave Background (CMB) makes it "difficult to doubt that the universe has evolved from a hotter denser early stage". In ...
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Can the metric expansion of space theoretically be exploited to do work?

Other questions tackle why space doesn't expand matter, and I've read their respective top answers, but I'm interested in something a little different. Let's say I set up two masses that are 1 Mpc ...
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Is the universe expanding, or is everything in it getting smaller?

I was recently reading some of the nice answers to the (related) question: If space itself is expanding, how could we ever tell? From many of the answers their appears to be a symmetry between: A ...
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What does the cosmological constant do? [duplicate]

I think just about anyone who has studied Einstein or relativity has heard of the cosmological constant. He added it so that his equation would return an universe that is static. Of course, the ...
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Time evolution of radiation (black body) spectrum in expanding universe

I was looking at Peebles' paper on recombination (https://ui.adsabs.harvard.edu/abs/1968ApJ...153....1P/abstract) and I don't understand one of the key equations he uses in his derivation of the ...
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What is the meaning of Hubble tension?

The people say that the value measured for the Hubble constant for the CMB is 4.4 $\sigma$ away from the value measured by Type Ia supernovae. I don't get it The value for CMB is $H_{0} = 67.4 \pm 0....
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Jeans Instability in an Expanding Universe, Understanding the perturbation

I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding universe. There are 4 equations but I think exercising on one of them would be helpful for me. So ...
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What is cosmological critical density?

I'm taking a first course in cosmology and I'm struggling to understand exactly what critical density is. I've been told "Critical density is the energy density of the universe that corresponds to the ...

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