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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Is the Universe being dragged into a singularity rather than expanding?

Recently I've read that the expansion of the Universe as observed is not exactly uniform as proposed by Hubble. Therefore is it possible that instead of space expanding in all directions all matter is ...
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60 views

How the mass is conserved during continuous expansion?

If universe is continuously expanding doesn't it contradict the fact that mass is conserved? Because if mass is conserved so how is universe expanding?
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204 views

The Big Bang theory hypothesis

Is there a simple way to state the hypotheses of the Big Bang theory? I have the impression that the Big Bang singularity is merely a consequence of Freedman equations. Could somebody clarify what ...
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248 views

Why is the Baryon Acoustic Oscillations (BAO) calculated like this?

In the Wikipedia, it says that, when calculating the Baryon Acoustic Oscillations (sound horizon), we measure $150\text{ Mpc}$, saying that the sound horizon is the "Physical Length of sound horizon ...
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53 views

Does the total particle energy increase in FRW Universe?

If a particle travels on a geodesic with 4-momentum $P^\mu$ in a spacetime with a Killing vector $K_\mu$ then we have a constant of motion, $K$, given by: $$K=K_\mu P^\mu$$ Using the relationships: ...
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56 views

Is the Big Bang notion compatible with the flat space? (A problem with the **moment** of Big Bang, not with the place of it) [duplicate]

A passage from a paper: "If one imagines running the clock backward in time, any given region of the universe shrinks and all galaxies in it get closer and closer until they smash together in ...
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53 views

Is there any explanation for these objects apparently traveling faster than light? [duplicate]

This is from a documentary on the science channel: https://www.youtube.com/watch?v=RaELad94KZs
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57 views

Meaning of $R(t)$ in FLRW metric

In the FLRW metric what is the meaning of $R(t)$ from a geometric point of view? And from a physical point of view? $$ds^2 = dt^2 - R^2(t) \left( \frac{d\bar r ^2}{1-\kappa \bar r^2} + \bar r ^2 d ...
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53 views

Hubble Constant at understandable distances

If I had two tennis balls, one meter apart, at what velocity are they moving away from each other due to Hubble flow? (Assume they are in a zero G environment, and nothing impedes their movement.)
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1answer
101 views

Where is the universe expanding from? [duplicate]

It seems to be quite a common question to ask "what is the universe expanding into?" However I want to ask "where is the universe expanding from?" Is there a "centre of expansion" of our universe ...
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1answer
64 views

Was the expansion after the big bang faster than the speed of light? [duplicate]

I think it's called Planck time and it's the speed at which matter spread during the big bang. Was the big bang expansion faster than the speed of light?
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68 views

Is the light from the stars within our galaxy redshifted?

I know that metric expansion does not happen within a galaxy. But is the light from stars in our galaxy somehow redshifted? Also, everyone says that our galaxy isn't expanding. But is there a slight ...
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534 views

How is “little $h$” measured in cosmology? The dimensionless parameter from the Hubble constant, $H_0$

Hubble's law has been well-know for close to a century now. It is written as $v = H_0 d$ where the Hubble constant $H_0$ is the constant of proportionality between recession speed $v$ and distance ...
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165 views

What is the implication of an expanding universe on a small scale? [duplicate]

As a layman I am struggling to understand what effects the fact that the universe itself expands has on distance, density etc. If the universe expands by a factor $k$, does that mean that the ...
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2answers
142 views

Do clocks measure conformal time (new argument)?

Assuming the spatially flat FRW metric for simplicity: $$ds^2=c^2dt^2-a(t)^2(dx^2+dy^2+dz^2)$$ where $t$ is cosmological time, $a(t)$ is the scaling factor and $x,y,z$ are co-moving spatial Cartesian ...
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1answer
72 views

What proof do we have that space is expanding? [duplicate]

how can we really say that space is expanding when we cannot see the distant galaxies forming nor are the planets nearby newly formed, nor is anything expanding in size nor are we going far away? ...
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64 views

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 ...
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95 views

Do we really need inflation for horizon problem?

This is a very fundamental doubt. I think i am missing something in inflation. The inflation theory solves the horizon problem by stating all of the universe started from a very small region in ...
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1answer
174 views

Traveling faster than the universe is expanding?

I was thinking, if the universe is space and time expanding, lets say in 3D (like a sphere). And it is doing that in a constant or not constant speed, there is a debate about that, never mind that but ...
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352 views

What does the accelerating expansion of the universe mean about its end?

So we know that the expansion of the universe is accelerating.When we thought that the expansion of the universe was slowing down,we predicted that it would reach a plateaux and maybe reverse the ...
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2answers
66 views

We stretch as the universe does [duplicate]

Here's an idea I thought about; more of a thought experiment than anything: The universe is essentially the same as the universe 14 billion years ago, just stretched. However, all matter stretches ...
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1answer
66 views

Why is a negatively curved universe older than a flat one?

And a positively curved universe younger than a flat one? I get that negatively curved means an open universe, and positively curved is a closed one. But how can this affect their age? Is it ...
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1answer
149 views

Age of the universe [closed]

Does the fact that the edge of our observable Universe contains information from the beginning of the Universe give us an ability to determine its age? The edge of our observable universe surely has ...
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35 views

Why do we say the universe is expanding faster and faster [duplicate]

The further we look away, the more redshift and expansion. But isn't looking further away looking back in time? Why do we not say that the younger universe (further away) was expanding faster and ...
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66 views

Does conservation of moment hold in space expansion?

I recently watched a video describing that the expansion of space is a "stretch". Everywhere in the universe expanding together. My question is, does conservation of momentum hold true in this ...
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2answers
199 views

Where did energy come from?

So it is my understanding that before the big bang there was a very small point that held all of the matter in the universe. My question is what form did energy of the universe take on when it was in ...
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Why haven't we seen the big bang?

The Andromeda galaxy is 2,538,000 light years away, so if we view Andromeda from a telescope, we see Andromeda how it was 2,538,000 years ago. Now the diameter of the visible universe is 92 billion ...
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When they say the universe was the size of a baseball about a billion billion billion billionth of a second

after the big bang. Does that means the observable universe was the size of a baseball, or does it mean the entire universe? I'm guessing it means the observable universe - as we really don't ...
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2answers
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Why should I believe that matter does not expand along with the space between it [duplicate]

This question has been asked before under other guises. I am not a scientific profesional however i have some schooling in pyhsics and mathematics and have a keen interest in these subjects. It seems ...
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252 views

Does the universe have a different age to different observers?

Although I understand that the universe is approximately 12 billion light years wide, and that may mean that it took the light photons 12 billion years at 186,000 miles per second to reach the ...
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2answers
382 views

Where is all the Dark Matter? Theoretical Question

In my Physics Class we had to look into possible areas where Dark Matter could be "hiding." Such as Black Holes and so on. Dark matter really can be "seen" through its gravitational effects, and we ...
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2answers
117 views

Expanding universe - does the Hubble red shift come from gravity? [duplicate]

Measurement of the expansion of the universe is based, in part, on the red shift of light. The speed of light is a constant, although some people argue that it may be changing over time, I am going to ...
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Why is the singularity of the Big Bang not considered to be the center of the Universe? [duplicate]

If the universe is expanding, then at some time in the past, it must have started from a single point but why this point is not the center of the Universe. Just like the singularity of black holes is ...
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1answer
115 views

Light clocks measure conformal time - detailed argument

Let us assume that an observer is stationary at the origin in expanding space. We assume the FRW metric near the origin is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Let us assume that the observer ...
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1answer
77 views

Is our locally measured time actually conformal time?

The FRW metric at the origin $r=0$, with $c=1$, is given by: $$ds^2=-dt^2+a(t)^2dr^2$$ Now one can change variables so that near the origin the FRW metric is approximated by the Minkowski metric ...
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1answer
93 views

Flat universe and accelerating expansion

As current data suggests that the universe is flat, a flat universe would imply a slowing expansion rather than an accelerating one which we observe. Neither in an open nor a closed universe there is ...
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1answer
99 views

Are we really sure that the whole universe is expanding?

Haven't we thought about the possibility of only a region of the universe is expanding? What about space is expanding in this region and space is compressed in another region, that satisfies the ...
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1answer
75 views

Does time expand along with space? [duplicate]

The flat FRW metric is given by: $$ds^2=-c^2dt^2+a(t)^2dr^2$$ If we take $dt=0$ then we get: $$ds=a(t)\ dr$$ Thus we find that space expands. If we take $ds=0$ to find the null geodesic followed ...
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Are atomic energies increasing as the Universe expands? [duplicate]

Starting from the FRW metric (for simplicity flat space, radial direction only): $$ds^2=-c^2dt^2+a(t)^2dr^2$$ If we take $dt=0$ then the proper distance $ds(t)$ between two spatially separated ...
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2answers
47 views

Is there a friction or retarding force in creating spacetime?

As matter and energy expand with the big bang, they create spacetime (or so I've been told). Is there any evidence that this creation of spacetime has a retarding force on the expansion? If not, ...
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Big Bang…or…Everywhere Stretch?

Recently I watched a minute-physics video that suggested that a better name for the beginning of time would be "Everywhere stretch" because there wasn't a space-time singularity that formed where the ...
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1answer
310 views

Is dark matter moving faster than light? [closed]

My way of thinking is as follows: Consider a balloon. Let this be blown into a nearly elliptical sphere. Now, the temperature inside the balloon is increased by some means; this leads to the ...
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2answers
72 views

Expanding universe and speed of light [duplicate]

Galaxies are moving away from us proportional to the distance between us and them , but nothing can travel faster than light, so even the farmost galaxies should be travelling away from us along with ...
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2answers
81 views

Why do we form cosmological theories based on old data?

Since the light we receive from distant galaxies may be between 7 and 14 billion light years away, the redshift we see indicates that the universe was expanding at that time (7 to 14 billion years ...
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Super-luminal separation

Suppose there are are 5 bodies separating in space (due to it's expansion), and let us consider that they can emit light. 1st body moves in the -x direction, at the speed of light. 2nd body moves ...
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43 views

Cosmological constant theories

Are there any hypotheses on the expansion of the universe, which have a cosmological constant, or some other parameter, that results in a universe with an ever increasing rate of acceleration for the ...
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1answer
88 views

Redshift of supernova light curve

I am trying to understand how the width of a supernova light curve depends on the redshift of its component frequencies. Let us make the simple assumption that the light curve is Gaussian. The ...
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2answers
245 views

How confident are we that mass is not being lost in the universe?

After reading about the latest super-massive black hole in Nature 518, 512–515 (26 February 2015), I couldn't help but wonder if the accelerating expansion is a result of mass being lost. My ...
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2answers
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How does a particular galaxy's apparent recessional velocity CHANGE as the universe expands?

By this I mean will the light from distant galaxies continue to show red shift (i.e. if you look at light from a particular galaxy now and in 10000 years time, the spectra will have been shifted ...
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1answer
172 views

Do we need Dark Energy to explain the acceleration of the expansion of the universe?

As far as I know is the discovery that galaxies that are farther away are moving faster from us than galaxies the are closer by. This led to the theory of Dark Energy. The acceleration was speeding ...