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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1answer
450 views

How to find the density of electrons during the recombination era?

I am supposed to find the value of the redshift during photon decoupling when $\Gamma=H$, where $\Gamma=n_e \sigma c$ is the rate of photon-electron scattering and $H$ is the Hubble parameter. I know ...
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Gravity as a repulsive force

As we know that universe is expanding, why can't we think the gravitational force as repulsive in nature in the realms of dark matter and dark energy?
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“Cosmological constant problem” and dark energy

Simply stated: is the "cosmological constant problem" (the discrepancy of about 120 orders of magnitude between measured values of vacuum energy and the ones predicted by Quantum Field Theory) in any ...
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2answers
559 views

A fundamental problem with the cosmological principle?

The cosmological principle states that: The spatial distribution of matter and energy in the universe is homogeneous and isotropic when viewed in a large enough scale. I find this notion ...
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What is the future of the CMBR?

Galaxies farther away than let's say z=2 are beyond our reach. If we today receive a message from such a galaxy, our response message will never reach back to them. So goes the current knowledge. This ...
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4answers
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What is a layman explanation for why the wavelength of light increases when space itself expands?

I understand that the universe is generally agreed to be expanding based on observations. What I have read is that space itself is expanding. My question is why does this expansion of space affect the ...
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Can anyone explain the unit for rate of expansion of universe?

If you google for 'what is rate of expansion of universe' you get Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 miles) ...
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What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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If light speed determines the rate of time, could the speed of light be varying over time and we be unaware of it?

Is this a valid implication/alternative explanation of universal red-shift? I thought I'd ask this when I read this question about light clocks. I had speculated this way: If light speed determines ...
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Could the spatially flat universe start small?

The global space has been measured flat within a small margin of error. According to FLRW, the flat universe has always been infinite. At the time of the Big Bang the universe was infinitely large ...
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The limit of the Universe that we'll ever be able to see

After watching Neil Turok's talk at 13:00 - 13:20 he says that the Universe expands and there is a limit beyond which we will never be able to see anything. And that's due to the Dark Matter which (...
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Expansion of the universe – impossible for future astronauts to make it back

The universe is rapidly expanding. If future astronauts built a spaceship that could travel close to the speed of light and took off, there is a certain point where they could not make it back again ...
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Does the age of the universe depend on the way it is defined?

I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that: ...
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Why physics cannot affect cosmological modes whose wavelength is larger than the horizon?

I started to learn cosmology recently. Confused about this point in a book. The book is Modern Cosmology by Scott Dodelson. I am confused about this in Chapter 6 "The horizon grows as the scale ...
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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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1answer
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Is the observable universe equivalent to 'our' light cone?

All the objects we can observe (stars, galaxies, ...) must be in our past light cone, since otherwise we couldn't see them. Presumably there are more objects located outside of our observable universe ...
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Is the acceleration of the expansion of the universe caused by expansion of spacetime itself?

When considering the accelerating expansion of the universe, it is often stated that dark energy is required to cause acceleration of expansion in order to overcome the pull of gravity. However, it ...
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If inflation is an exponential expansion, is the exponent known?

Inflation is often called exponential expansion. I can't seem to find what that exponent is. Is it 2, or 20, or completely unknown?
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'My question is about' …the size of the Universe after Inflation [duplicate]

Hitherto, I had assumed, on account of Inflation, "boom" the Universe was "out there" and it has taken light 13.6 BY to get here. However... Quoting the aerospace engineer: In my thinking if the ...
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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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Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
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Forces between adjacent particles during big crunch

Ignoring the effects of gravity, consider two adjacent objects/particles in contact. If the universe was expanding, this would cause them to separate at $\vec{v}=H_0 \vec{r}$, where $H_0$ is the ...
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Is the “cosmological constant tension” the prime reason that we believe the expansion of the universe is accelerating?

Background: For a few years now there is significant difference in the calculation of the cosmological constant between standard candles methods and CMB methods. CMB measurement comes from a very ...
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If the universe expands does spacetime,dark energy,dark matter,gravity expands also?

If the universe expands does spacetime,dark energy,dark matter,gravity expands also ? Do forces,energy/matter expand with the universe also ? When we say that the universe is expanding ,what is ...
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How can I compute the scale factor for an open universe?

I'm struggling in obtaining an analytical expression for an open-universe $a(t)$. I know it is usually calculated from the second Friedmann equation, and with respect to the image I know how to ...
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Is the energy density of vacuum uniform in the whole universe?

This is not a duplicate. I am not asking why the energy density of quantum vacuum stays constant while the universe expands. My question is about how we know/measure that this vacuum energy density is ...
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Does $G$ effectively change in an expanding metric?

Assume that we have a flat FRW metric expressed in conformal time $\eta$ so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where $a=1$ at the present time $\eta=0$ and the ...
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Could decaying dark energy solve the Hubble tension? [closed]

The redshift-based distance ladder values of the Hubble constant are higher than that inferred by the cosmic microwave background. This discrepancy in the last few years has become highly ...
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Does space expansion affect the CMB photon frequency?

I have read this question: Effect of expansion of space on CMB where Ted Bunn says: For definiteness, let's consider a wave packet of electromagnetic radiation with some fairly well-defined ...
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Intergalactic medium and cosmic expansion

Presumably, the intergalactic medium (IGM) should get less dense as the universe expands. Also presumably, There should be a Hubble redshift to absorption lines due to the IGM. Could this provide ...
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Why does Penrose expect visible Hawking Points in CMB?

I've only just started reading into Conformal Cyclic Cosmology. My understanding of the hypothesis is that, with some conformal transformation of space, the very end of the universe at $t=\infty$ ...
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Is it possible that the universe is not expanding faster than the speed of light? [closed]

What if the speed of light increases proportionately to the expansion of space? Is it possible that light traveling in the medium of space gets to travel faster if the medium is expanding? An analogy ...
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Why is the measure problem an issue if the universe is always finite?

As a follow-up from this question, I was wondering how the measure problem applies to the idea of eternal inflation. First, why is it even a problem? As time progresses, more universes are created in ...
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How necessary is inflation given the laws of physics?

How deeply is cosmic inflation required for the laws of physics? Is inflation required for a universe remotely like ours, or is it simply a contingent on the starting conditions of the universe? For ...
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Inflation v.s. Big Bang

It is said that that transition or certain quantum rotor field, and “rolling” down into the valley (of some quantum potential), is what causes inflation to come to an end, and create the hot Big Bang. ...
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COSMIC ACCELERATION

Please forgive a probably naïve question from a lay person. I understand that galaxies are all moving away from us in every direction we look. And for any given distance the speed is the same. And ...
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How to Change Coordinate Systems in General Relativity

Let me preface by stating that I have no experience with General Relativity. I am working on a project for school that requires a little knowledge of it, so I am hoping to find some help. I do have ...
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Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?

In Newtonian gravity, an infinite volume filled with a uniform distribution of mass would be in perfect equilibrium. At every point, the gravitational forces contributed by masses in one direction ...
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Is the expansion of the universe uniform? Are there parts which are expanding faster than others?

Is the expansion of the universe uniform? Are there parts which are expanding faster than others? The very fabric of the universe is expanding..but is it expanding at a constant rate? If it is not ...
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Does curvature affect the change in the acceleration/deceleration of the Universe?

Suppose I'm modelling a Universe with non-zero curvature, filled with matter, radiation and dark energy (further described by quintessence). The appropriate Friedmann equation would be of the form: $$...
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1answer
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Momentum conservation in FRW spacetime

The spatially flat FRW metric in Cartesian co-ordinates is given by: $$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$ As I understand it, since the metric does not depend on the spatial co-moving co-...
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If time slows in the presents of gravity, how does the conclusion that the universe’s expansion is accelerating account for this?

I am sure there is a simple explanation for this and I’m not a physicist so hoping someone here can help answer this for me. Time is retarded by gravitational field. Expansion in all directions ...
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Could the matter for a Big Bang come from the Hawking radiation of a super fast expanding universe?

If I understand it right, the universe will at some point in time expand so fast, that from any particular point in space, an observer will see an event horizon around himself. This event horizon ...
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Is the expansion and contraction of the Universe an endless repeating cycle due to Newton's 3rd Law of Motion? [closed]

If the creation & expansion of the Universe was an action, then the reaction to this action is that the Universe will contract and be condensed back into a singularity. If the contraction of the ...
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Why does the expansion of space lead to cosmic redshift?

I have always thought of light as a point-like object in space, with the EM field strengths dependent on time. However, stretching of space would not affect a point-like object. Thus, I assume that ...
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Fraction of the Universe that is unobservable

I was watching this talk by Leonard Susskind. Near the end of his talk, he mentions how, because of the accelerating expansion of the Universe, there is a horizon to the Universe beyond which we ...
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FRW co-ordinate system expanding with the Universe?

The flat FRW metric using standard cosmological time $t$ and Cartesian spatial co-ordinates $(x,y,z)$ (with $c=1$) is given by: $$ds^2=dt^2-a^2(t)(dx^2+dy^2+dz^2).$$ I understood that the FRW metric ...
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How does the cosmological constant dominate cold matter perturbations?

I have been studying inhomogeneous universe in physical cosmology, whereupon I arrive at cold matter perturbations which are dominated by various background fluids. I understood the solutions for the ...
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How do Graviton-based theories of gravity explain the expansion of the universe?

In General Relativity, the expansion of the universe is modeled using the Friedmann–Lemaître–Robertson–Walker metric, and the expansion itself is a metric expansion by which the scale of space itself ...
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How do we determine both the distance to and speed of distant stars?

My understanding is that we can measure the distance to far away stars by using their color to infer the absolute magnitude of the light they emit and compare that to the observed magnitude of the ...