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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Does matter follow the expansion of space?

Non-physics person here. I am trying to understand the expansion of space. I've read about the redshift of light and the balloon model (2 points on expanding balloon move away from each other). The ...
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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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How far away must a galaxy be for its light never to reach us due to the expansion of the universe?

My understanding is that at the present rate of expansion of the universe some galaxies are growing more distant from us at such a rate that light from them will never reach us. My question is how far ...
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Does time expand with space? (or contract)

Einstein's big revelation was that time and space are inseparable components of the same fabric. Physical observation tells us that distant galaxies are moving away from us at an accelerated rate, and ...
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How do you extract cosmological constants like $k$ or $H_0$ from the fluctuations in the CMBR (WMAP)?

I am currently reading a book about Astrophysics and also tried to find some information about it on the web, but was not able to figure it out. That is why I ask you guys now. Any help is greatly ...
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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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Time dilation of distant cosmic events. What is it?

I was reading the Wikipedia's page about "tired light", where we read that any alternative explanation to the observed redshift (described by Hubble's law) should be able to overcome several ...
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Is there any way to tell if the Big Bang happened everywhere or just in some regions of the entire universe?

On large scales, matter seems to spread uniformly in our observable universe so we think the Big Bang happened everywhere in the observable universe. Is there any way to tell if the Big Bang happened ...
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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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Present vs Past in Cosmological time

My question is in regard to “seeing back in time” when observing objects in space. I understand the concept that light takes a certain amount of time to reach the earth and we can determine 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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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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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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If universe expands without limit but dark/vacuum energy density remains the same, then as space time expands, more of that energy is created?

And if it is the case, does it mean that as universe expansion has no limit the energy that can be created is infinite and therefore there is infinite potential energy?
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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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Fulvio Melia's linear cosmology

I would be interested in what people think of Fulvio Melia's argument for a linearly expanding Universe. I realize that the experimental evidence seems to be pointing to an accelerating Universe but ...
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Is the inverse of the Hubble constant always approximately the age of the Universe?

So I know from class that $\frac{1}{H_0}$ is an approximation for the age of the Universe, where $H_0$ is Hubble's Constant. Now let's say that a large amount of time passes - this means that Hubble's ...
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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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Hubble radius and communication between two observers

According to Dodelson, Modern Cosmology (p.146) There is a subtle distinction between the comoving horizon $\eta$ and the comoving Hubble radius $(aH)^{-1}$. If particles are separated by ...
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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_\...
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Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy?

Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy? The accelerated expansion only tells us that the Einstein field equation must contain a ...
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Conversion between extragalactic distances

What is the conversion between redshift $z$ and lightyears? When an article says that some object is at a distance of $z = 1$, then what distance are we talking about in light years or parsec?
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Furthest distance I can travel and still be able to get back to Earth?

The expansion of the universe currently makes anything beyond ~16 billion light years away from us unreachable (because of the cosmic event horizon). This makes less than $3\%$ of the observable ...
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Solving the Friedmann Equation for a specific universe

Suppose instead of the cosmological constant there is a quantum field with equation state parameter $w = -1/2$. Assume also it is a flat universe with only this quantum field ($Q$) and with non ...
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What would happen to de Sitter universe if the cosmological constant disappears?

De sitter universe is dominated by a cosmological constant (devoid of matter and radiation), corresponding to dark energy in the far future or the inflaton field in the early universe, leading to an ...
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Do free particles slow down in expanding universe?

My favorite toy model of cosmic microwave background in the expanding universe is a cube with totally reflective walls. If CMB is a black-body radiation, it must be isotropic and homogeneous - the ...
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Expansion of the Universe: is new space(time?) being created or does it just get stretched?

Is new space(time?) being created as the Universe expands, or does the existing spacetime just get stretched? If it just gets stretched, why do galaxies move along with the expansion instead of just ...
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Very Basic Hubble Constant Question

In an article titled "Gravitational Waves Could Solve Hubble Constant Conundrum" Data from the cosmic microwave background suggests the universe is currently expanding at a rate of about 41.6 miles (...
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Does the Universe itself move to any direction on top of expansion?

So the universe expands right, but does this expansion move to any direction in hyperspace as an expanding cluster of matter somewhere? It's like if you would explode a bucket of paint under the. ...
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How Can the Universe Be Bigger Than It is Old? [duplicate]

My question is this: If we can look back to roughly 500m years after the big bang and have estimated the age of the universe at 13.7B years, why is the galaxy we are looking at not a part of the non-...
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$\rho(t)\gt or \lt \rho_{critical}(t)$ depends upon $k$ for expansion or contraction in cosmology?

From friedmann equation $$1=\frac{\rho(t)}{\rho_c(t)}-\frac{k}{a^2H^2},$$$$\dot a(t)=+-\sqrt\frac{k}{\frac{\rho(t)}{\rho_c(t)}-1}$$ for $k\gt0$,$$\rho(t)\gt\rho_c(t)$$ and for $k\lt0$,$$\rho(t)\lt\...
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Could gravitational waves give an illusion of an expanding universe? [closed]

‪Could the effect of “expanding” gravity (gravitational waves spreading at the speed of light) as it travels through space (and therefore becomes less local) red-shift all EM waves and thus give us ...
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Why is the hypersphere not seriously considered by cosmologists as the best model for the overall shape of the universe?

Cosmologists seem to not seriously consider the hypersphere as the best model for the universe even though they mention it as a candidate from time to time. If you look closely, it seems to be a very ...
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Traveling to another galaxy impossible for ever?

If galaxies are moving apart from each other faster than the speed of light, does that mean that it's impossible to travel to another galaxy, regardless of what technology get discovered? Because you ...
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How the cosmic inflation solves the horizon problem (an exact solution)?

I am reading an article, Inflation and CMBR by Charles H. Lineweaver. https://www.mso.anu.edu.au/~charley/papers/canberra.pdf (Page 5/13) He explains the inflation period as the shrinking of the ...
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Can the fabric of space-time be contoured into hills instead of just wells?

Einstein's general theory of relativity states that gravity is the distortion of space-time into gravity wells. In order to illustrate this, a flat plane is used to represent undistorted space-time ...
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By What Mechanism Does the Universe Expand in Loop Quantum Gravity Theory?

So, the universe is expanding, and, from what I've heard, that expansion is accelerating. However, I'm unclear as to the specific mechanism involved, particularly as would be described in a Loop ...
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$z$ (redshift) at the inflation start up time?

The $q$ parameter is at present -ve which implies accelerated expansion , hence we can approximate our universe now to be dominated by dark energy since dark energy provides exponential growth $~e^{\...
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Spacetime diagram of LCDM

I have a question concerning the LCDM spacetime diagram https://i.stack.imgur.com/Uzjtg.png published on the Physics Forum Stack Exchange website Can space expand with unlimited speed? How are the set ...
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Expansion and photon decay

We know that there are two types of photon "decay", not destruction, but loss of energy and resultant lengthening. ONe type is due to great gravitational pull and the other is due to the expansion of ...
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How likely are Primordial Black Holes to form in the early universe?

In the very early universe, tiny fluctuations created slight over- and under-densities in mass/energy. As far as I understand, if a region becomes sufficiently over-dense, a primordial black hole (PBH)...
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Metric to describe an expanding spacetime from coordinates reflecting the perspective of a local observer

The FLRW metric describes the metric expansion of spacetime from the perspective of comoving coordinates. Given the way this metric is usually formulated, comoving distances stay constant, and the ...
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Luminosity distance

If light is emmited from galaxy A at $t_e$ and received at galaxy B at $ t_o.$ The flux at B is $$ (1) -- \phi(t_o)=a^2(t_e)L(t_e)/4πd_L^2$$ $d_L$ being the current distance between A and B In the ...
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Does space expansion mean the escape velocity at a black hole event horizon is strictly less than $c$?

The title pretty much sums it up. I could ask this question slightly differently; does the expansion of space cause the effective event horizon to shrink below what it would be for flat space? If so,...