Questions tagged [expansion]

Questions about the increase with time of distance between two distant points in the Universe.

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1answer
92 views

Expansion of the universe speeding up

I know that the expansion of the universe is speeding up, but is there an equation to measure this? Also, what is the cause of this?
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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 CMB makes it "difficult to doubt that the universe has evolved from a hotter denser early stage". In my understanding, CMB is just a ...
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1answer
66 views

Why is constant velocity drift not an accepted explanation of Hubble expansion?

Edward A Milne at the Oxford University proposed that what Hubble was observing was simply a natural sorting of random galactic motions. If a group of galaxies formed together moving at various ...
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1answer
62 views

Expansion of the universe and visibility of distant galaxies [duplicate]

The universe expands at an accelerating rate, which makes me wonder if at a certain point the speed of expansion between two points will exceed the speed of light. In that case, it might eventually ...
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50 views

Any theories about universal expansion due to action at the universal event horizon?

I'm curious if there are any theories about the increasing rate of the expansion of the universe that are centered around gravitational pull from matter outside the event horizon of either the ...
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1answer
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Does the gravity oppose to the Universe expansion?

I'm trying to understanding the very basic of the current cosmology. I searched in the old questions but I found nothing that answers specifically to my questions. As far as I understand, we have to ...
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118 views

Scale factor 0 for expanding universe

Does the FLRW model have a scale factor $0$ at the singularity, meaning that even infinite distances become $0$ there according to the mapping $x \mapsto a(t)x$, for $a(t)=0$? I am asking this ...
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1answer
111 views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=v/c$ won't apply)? [The ...
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2answers
98 views

Why does the Supernova 2006cm give a very different value for the Hubble constant? Why doesn't it increase error bars for the Hubble constant?

The Supernova 2006cm has a redshift of 0.0153 which translates into a recession speed of 4600 km/s. It has a distance modulus of 34.71 which translates into a luminosity distance of 87 Mpc. This ...
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3answers
148 views

Is the eventual heat death of the universe due to the expansion of the universe?

I am reading about the accelerating expansion of the universe and something struck me. The observation that things further away were accelerating away faster and that early observations made it look ...
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3answers
223 views

Cosmological redshift vs doppler redshift

I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift. He ...
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1answer
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Why do we expect Type Ia Supernovae to be brighter in a decelerating Universe than what has been observed?

This seminal review paper in the PNAS by Robert P. Kirshner (https://www.pnas.org/content/96/8/4224) says the following: If the universe had been decelerating—–in the way it would if it contained ...
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148 views

Do gravitationally bound (e.g. orbiting) objects really resist the expansion of space?

Thinking about the expansion of space, it is commonly explained that gravitationally bound objects (like galaxies and galaxy clusters) will not be affected by the expansion of space (dark energy). I ...
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1answer
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Difference between 3D real space and 3D redshift space?

With regards to cosmological simulations (e.g., N-body dark matter only simulations), I've seen people talk about the positions of objects (halos, galaxies) in the simulation in terms of 3D real space ...
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1answer
96 views

Can expansion of space blueshift galaxies with respect to one another?

Suppose we have two galaxies in an otherwise devoid space, and suppose the galaxies are gravitating. Suddenly, dark energy kicks in, and accelerated expansion of space begins ... (1D force diagram. ...
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1answer
103 views

Universe expansion

I'm reading the book "The theory of everything". In the second chapter it says the Universe is expanding, and we know this because the measurements made using the doppler efect to other galaxies gives ...
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1answer
126 views

Cosmological redshift - How do we know it's not caused by the observer's time dilation?

Instead of cosmological redshift being caused by the metric expansion of space, why couldn't cosmological redshift be caused by our perspective moving faster in time? Said another way, if the ...
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1answer
237 views

Will intergalactic travel be impossible in roughly 15 billion years?

Even if we (or aliens somewhere in the universe living at that time) somehow invent something that can go just short of the speed of light, will intergalactic travel still be impossible in roughly 15 ...
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1answer
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If the space is expanding near the speed of light or the speed of light, why Andromeda is approaching or coming near to our galaxy (Milky way)? [duplicate]

Space with distance of 1megaparsec is expanding with rate of 70km/s, and if space with a distance of 4296 megaparsec is expanding at rate of 300,720km/s, then why Andromeda is coming to us? Even there ...
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Expansion of space or constriction of length scales?

According to wikipedia, "Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. Although ...
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2answers
111 views

Does the expansion of the universe increase with distance?

I read something in a book the other day which has gotten me really confused. They were talking about the expansion of the universe, and if I understood it all, they said that the expansion of the ...
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1answer
203 views

Has the Big Crunch been ruled out?

If the dark energy equation of state $\omega$ evolves with time, and becomes larger, this would result in decelerated expansion and eventually a Big Crunch. If $\omega$ decreases with time, becoming ...
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How are constraints on dark energy improved the most?

Today we have various data available from independent probes to constrain dark energy, i.e. its equation of state $w$. These include Type 1a supernovae, BAO, CMB, large-scale galaxy surveys, etc. Are ...
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1answer
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How is the expansion rate of the universe obtained from Baryon Acoustic Oscillations?

I understand that we can measure the sound horizon from the anisotropies in the CMB. Large galaxy surveys today can construct two-point correlation functions, which reveal peaks at distances equal to ...
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how to define the universe itself is expanding greater than than the speed of light?

since we observe galaxies and other steller objects in very tiny tiny scale, and how we can understand that the speed of expansion on universe is higher than the speed of light ? even using the ...
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1answer
168 views

Why hasn't the CMB already receded away faster than the speed of light?

If there are supposedly galaxies, or whole other parts of the universe, that we can't ever see due to them receding away from us faster than the speed of light, then why isn't or hasn't this occurred ...
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2answers
529 views

Gravitational red shift vs Doppler redshift: Is the universe really expanding?

Is it possible that the redshift observed by Edwin Hubble is really from a gravitational redshift and the universe isn't expanding as he has predicted? What I think I know thus far is this: Redshift ...
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Is it possible for redshift to be caused by light being somehow slowed down during travel? [duplicate]

In other words, is the assumption, that redshift is caused by universe expansion, definitive? Or are other theories still being considered? Could it be, that light is being slowed down by dark energy, ...
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Expansion of space - how do we end up with an unobservable region?

I've just been answering a question on this issue,in physics SE,and realised that my answer conceals a point I don't myself understand. I understand that The expansion of metric isn't affected by ...
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1answer
86 views

Looking at the Expanding Universe through the lens of relativity [closed]

The basis and most observable metric for how the universe is understood to be expanding at an accelerating rate is the red shift in observable light. I am assuming it has been thought about or maybe ...
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1answer
65 views

What fills the new space created during expansion?

Does it remain void and pushes even more or will virtual particles eventually create normal matter?
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3answers
97 views

The expansion of the universe and how stars and galaxies still collide

How can it be that all galaxies and stars are moving away from each other yet they still collide? From what I have researched it as an open universe (or many open universes) yet the idea of stars and ...
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6answers
585 views

Are there other proofs of the expanding universe apart from the redshift?

The theory of the expanding universe is so widely accepted, that the redshift is sometimes used as a measure of distance to far away galaxies. But is it still possible that the redshift is caused by ...
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1answer
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Time-frame regarding cosmic expansion and the bound state of the Virgo Cluster

Due to accelerating cosmic inflation, other galaxies and galaxy clusters will eventually race toward the Hubble horizon apparently away from us, placing them effectively out of our sphere of influence....
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1answer
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Is the universe always expanding

Currently we observe that the universe is expanding. And this expansion even goes faster and faster. Why do we know (or presume) that this expansion started slowly at a singularity (i.e. big bang) ...
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1answer
83 views

Why Is The Expansion of The Universe Allowed To Move Faster Than Light? [duplicate]

Why is the expansion of the universe allowed to faster than light if everything that moves within a vacuum can't reach the speed limit of light? I've read that astronomers looked at the distance ...
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1answer
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From CMB anisotropy data observed in 1992, did astronomers figure out that the universe should be accelerating before its discovery in 1998?

CMB anisotropy was measured in 1992. I assume that astronomers, then, like now, would have been able to deduce the cosmological constant and things like that from the CMB anisotropy data. Then, from ...
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2answers
151 views

how do we know the universe is STILL expanding?

According to Hubble more distant galaxies are moving away faster but since the light took billions of years to reach us we should say they WERE moving away faster billions of years ago. How do we know ...
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1answer
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How can the universe be expanding? [duplicate]

I assume expansion implies an increase in the volume of the universe. But if the volume of the universe is increasing, does that not imply the area it is expanding into's volume is being reduced?
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1answer
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Expansion ofthe Universe [duplicate]

If the universe is expanding how come Andromeda is heading for us? In isolated spatial areas does gravity overcome universal expansion? What is the gravitational force between us and Andromeda?
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Is the acceleration of the expansion of the universe a direct observation or an interpretation?

Is the acceleration of expansion we observe with supernovae direct or is this an interpretation according $\Lambda$CDM model ? I mean what we observe is luminosity distance versus redshift, so to see ...
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2answers
701 views

How is the Universe's Expansion Accelerating if the Hubble Constant is Decreasing?

I am just getting into the field of cosmology and was wondering what it means practically to say that the Hubble Constant is decreasing while the expansion rate of the Universe is accelerating. I am ...
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2k views

When the universe expands does it create new space, matter, or something else?

I am wondering what exactly is meant when it is said the universe expands. Does it simply create new space for matter to fill, does it also create new matter/dark matter to fill that space, or am I ...
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1answer
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How to calculate how much a galaxy moves from its coordinate at redshift 0 to redshift 1?

I have two simulation snapshots in my hand at redshift 0 and 1. I know x, y, z coordinates of the galaxies in both redshifts 0 and 1, however there is no way for me to identify a single galaxy in both ...
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Influences from outside the observable universe explaining dark energy and expansion?

Could some influence from outside the observable universe be causing the expansion?
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Where can I find source data for the graph of metric expansion over time, as shown in this ESA web page?

This web page from ESA/Hubble, shows the size of the universe over time. (I know that "size of the universe" is a gross simplification and open to multiple definitions and ambiguity, but this kind of ...
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2answers
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Light beam (1 photon) in the limited universe?

If our universe is not infinite, what happens with the light beam (or photon) when it will travel through the whole universe? For example, observable universe according to wikipedia has diameter 93 ...
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2answers
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Does the edge of the Universe travel faster than the speed of light?

The Universe is expanding faster than the speed of light then does that mean matter at the edge of the universe is also traveling at the speed of light or are ether opposite sides of the universe are ...
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How can an infinite universe expand?

I understand the expansion of the universe as actually an increase in the ratio of space to matter. Is this a correct understanding? Otherwise, I don't understand how an infinite structure can expand....
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2answers
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Are galaxies expanding away from us at more than the speed of light? [duplicate]

If you go to the following video, at this certain time: https://youtu.be/Iy7NzjCmUf0?t=435 you will see that the narrator says: "We simply don't know what else is out there because the light from ...