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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How do we know that space expanded with speed faster that a speed of light during big-bang inflation?

How do we know that space expanded faster than a speed of light in inflation? I have read this Phys.SE question, and it says that limit for faster than a speed of light is for matter and waves only. I ...
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190 views

How does the Hubble parameter change with the age of the universe?

How does the Hubble parameter change with the age of the universe? This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to ...
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21 views

A physical understanding of the 2/3 in the Sachs-Wolfe effect

I am trying to develop an intuitive but quantitative understanding of the factor of 2/3 in the Sachs-Wolfe effect. I believe I have a picture that makes sense but it relies on one assumption I don't ...
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43 views

Friedmann equations question

Friedmann equations for critical density is: $$\rho_c = \frac{3H^2}{8\pi G}$$ Is there any other way to write this equation? For example: $$\rho_c = \frac{3}{8\pi GH^2}$$ I saw the above form on ...
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61 views

Is it possible to determine whether distant galaxies are gravitationally bound

In a previous question, one issue was related to the potential energy of cosmic structures. This raised in particular the question of whether these structures are gravitationally bound. If you ...
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1answer
37 views

Is the universe expanding on a preferred axis, or asymmetrically?

I remember back in 2011 i found this paper on the research a team of Chinese physicist completed about the universe may having a prefered axis when expanding. On ARXIV I haven't seen any other ...
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1answer
50 views

Measuring more accurately the distance of remote galaxies

From what I read in Wikipedia, the velocity of a Galaxy has two components: one is due to Hubble's law for cosmic expansion, and the other is the peculiar velocity of the galaxy. Since the peculiar ...
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1answer
49 views

Expanding universe space through matter or matter through space? [duplicate]

So the universe is expanding, rather space is expanding. By expanding we mean space is coming into existence at all points.  Is that an equal rate of expansion everywhere? Now the expansion does not ...
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1answer
14 views

Expansion of Galaxy's Caused by Electromagnetic forces

From my limited physics knowledge i know that gravity when compared to Electromagnetic forces is very weak... So if every object is polarized (positive and negative separation), then on a bigger level ...
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1answer
88 views

Expanding metre sticks [duplicate]

Given the Universe is expanding. Therefore everything within the U is expanding, in all dimensions, subatomic to cosmic. Then all metre sticks are expanding. The question: How can we measure U ...
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1answer
60 views

Thought experiment: Tethered galaxies - to the extreme

If two rocks were tied together with a tight, absurdly long, non-elastic rope, and placed on planets at either end of Earth's observable universe - or beyond - What would happen? Is the structural ...
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2answers
64 views

How do we observe the expansion of the universe?

This is my first question. So I was reading about expansion of the universe and from what I've seen, the only way that we know that universe is expanding is by measuring redshift. Also, here it says ...
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1answer
39 views

Can someone explain effects of time dilation on expansion of universe [duplicate]

I'm not a physics guy, but this is something I have always wondered about. If a clock flown by an object of high mass runs slower, what is the effect of this in regards to the density of the universe. ...
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Can expansion of space create energy? [duplicate]

In this question, expansion of space is considered, and it is explained that gravity and electomagnetic forces prevent matter from expanding. The idea I get in my head is something like a weight ...
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1answer
243 views

As the universe expands, the wavelengths of photons are stretched, and energy is lost. What about electrons?

Will electrons, and other particles, also loose energy as they travel through the cosmos? They have wavelengths. Do they get "stretched"? My guess is that the EM force, somehow, counteracts this ...
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2answers
90 views

Expanding universe and the speed of light [duplicate]

When physicists talk about the expanding universe they often say that the distant galaxies are not really "moving" away but instead the space itself between us and them is expanding. If this is true ...
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2answers
111 views

How does the gravity well change as space expands? [duplicate]

How does the gravity well change as space expands? If we assume that the Earth's gravitational field curves flat space to create a gravity well then how does the gravity well change as space expands ...
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25 views

extreme heat to extreme cold (define the endstate)

Contemporary cosmology frequently has space temperatures just after the 'big bang' in the regions of millions of degrees and with inflation and expansion of the universe this is now down to a couple ...
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3answers
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Does or should the metric expansion of space imply a locally observable increase in kinetic energy?

The title is the question. Here's why it seems like local kinetic energy should increase: Numerous questions and answers here and elsewhere suggest that the reason the metric expansion of space is ...
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1answer
43 views

Spacial curvature and expanding space

If we take the analogy that in an empty space the space is just a flat sheet then if there is a single planet or a star then the flat sheet will curve below the planet leaving a curvature shaped like ...
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1answer
57 views

How is CMB related to the temperature of the universe

As I understand it, CMB (cosmic microwave background) is the radiation emitted when matter decoupled at the early stages of the big bang. The thing I don't understand is do all stars emit this kind of ...
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2answers
387 views

Wouldn't the presence of dark matter slow the expansion of the universe?

If there is a huge element of dark matter in the universe, wouldn't this extra gravity prevent the accelerated expansion of the universe?
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2answers
52 views

Will the Universe eventually stop expanding

Sorry if this is a naive question, not being even a part qualified physicist in any way shape or form. I've read that the universe is expanding and the rate of expansion is increasing. The assumption ...
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32 views

Space within galaxies. Is it stuck by the gravity of the galaxy or expanding and “slipping past”? [duplicate]

I understand that the inter-galactic space is expanding but galaxies themselves are not. What is happening to the space within a galaxy? Is it fixed by the gravity of the galaxy or is it expanding and ...
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1answer
133 views

How far can something travel in a straight line?

Suppose you have an object some distance from you and moving at a velocity different to the Hubble velocity you'd expect at that point. How does the motion of this object change with time? Does it ...
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1answer
61 views

Conservation in space-time curvature

Pardon this possibly naive question. I'm starting to poke around in the topic of General Relativity (as soon as I can pull myself back up out of the vortex of underlying mathematics that I've gotten ...
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4answers
502 views

Expansion of the Universe, will light from some galaxies never reach us?

Is it true that the light from some galaxies will never reach us? The explanation for that is that the Universe expanding faster than the speed of light. But, if the speed of light is constant in ...
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What does a galaxy orbit?

Moons orbit planets, planets orbit stars, and stars orbit the center of a galaxy. So, my question is what does a galaxy orbit? The center of universe? (I know that the universe has no center)
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1answer
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Calculating Hubble's constant at earlier times [duplicate]

I want to calculate Hubble's constant at some redshift $z$. I have found the following formula: $$H^2=H_0^2\left(\Omega_m\left(1+z\right)^3+\Omega_{\Lambda}\right)$$ Now it's obvious that at higher ...
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1answer
82 views

What is the difference between matter & spacetime? [duplicate]

If the universe is expanding why doesn't the matter in it expand proportionally making it seem as if the universe is static? Alternatively, as spacetime expands why does it not just slide past matter ...
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1answer
91 views

How did the universe get so big so fast? [duplicate]

The universe started at the big bang around 15 billion years ago. The universe is now at least 92 billion light-years in diameter. Together, don't these mean that the universe, at some time in the ...
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2answers
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Does expanding space cost energy?

Does the cosmic inflation reduce the energy density (inversely) proportional to the volume, or does the inflation "cost" energy? Is space itself "something" created at the expense of energy?
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37 views

Cosmic Expansion - Why aren't we ripped off yet? [duplicate]

According to Hubble's law, the universe expands exponentially ever since the big bang. If the space-time expands, what effects does it have upon us, (1.Earth, 2.solar system and 3. Milky Way). ...
2
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2answers
73 views

Dark energy vs. gravity

If dark energy is everywhere around us, then why don't we get separated? For example why don't I get separated from the pen kept in front of me? Or take a similar example in free space. Is dark ...
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2answers
183 views

Rotation of Taylor expansion of a scalar

I have a scalar magnetic field in a volume expressed by the formula $$B(x,y)=B_0 + \frac{\partial B}{\partial x}(x-x_0) + \frac{\partial B}{\partial y}(y-y_0)$$ which approximates the ...
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Maybe photon energy is constant as the Universe expands?

This is a question following on from my previous post Time-like Killing vector in FRW metric? For simplicity I take the spatially flat FRW metric in cartesian co-ordinates given by: $$ds^2 = -dt^2 + ...
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2answers
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When was the cosmic background radiation in the visible spectrum?

From what I understand about the Cosmic Microwave Background Radiation is that it was from the big bang, and since space has streched and become bigger since then the wavelength of the CMBR has ...
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1answer
78 views

Estimating age of the universe by Hubble's law?

Hubble's law states that $v=Hx$ Age of the universe is calculated by $T= \frac{x}{v} = \frac{1}{H}$ but the velocity is not constant; it changes with distance, so I think that this equation cannot ...
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3answers
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Did “big bang” radiate light?

According to present day physics nothing existed before the "big bang", even space and time. But often in documentaries, the "big bang" is shown as a ball of light that exploded at some point. If this ...
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1answer
58 views

Measuring proper distance using a light beam

I wonder if someone can help me with the following problem. I send a light beam to a distant galaxy which then bounces back to me. I measure the travel time of the lightbeam using say a light clock ...
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0answers
48 views

If the universe is expanding then why does the distance between Sun and the Earth is not expanding? [duplicate]

It has been proved that universe is expanding in the accelerating way. If that is true than the space between the Sun and the Earth must also be expanding. But it is not so, why?
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Does dark energy have gravity?

So I keep running into this idea on the Internet that only mass has gravity, but isn't mass simply contained energy? Energy bounces around, or travels at the speed of light, because it is not in the ...
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1answer
56 views

Does a hydrogen atom today have same mass as a hydrogen atom in the future?

Does an atom of hydrogen today have the same rest mass energy as an atom of hydrogen a billion years in the future? Standard cosmology seems to tacitly make this assumption. But surely one can only ...
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4answers
111 views

Accelerating expansion of the universe: so?

Please help me out, I’m missing something. We know that, right now, space is expanding at roughly 73km/s/Mpc. This means: two points in space 1Mpc away from each other “move” 73 km farther away ...
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2answers
74 views

Metric expansion of space and Newton's second law

I wanted to ask the following question: If the expansion of the universe is really accelerating, does that mean a certain force applied on the universe? (According to Newton's second law) What kind ...
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1answer
34 views

Cheating to communicate beyond the horizons in expanding universe?

Let's say the Hubble constant is constant in the time interval we consider. So subjective "horizon" for any object has the same radius R. Let's say Bob and Charlie are just within the horizon of ...
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2answers
138 views

Time-like Killing vector in FRW metric?

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 there are Killing vectors in the $x$, $y$, $z$ directions implying ...
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2answers
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Can the cosmological constant change with time?

This post is a specialization of the post: Can the proportion of dark energy change? Can the cosmological constant change with time? If so, is there a measurement of this evolution up to now, ...
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How does the expansion of the universe not violate causality?

It is often said that faster than light travel would violate causality. However, because the universe is expanding, there are actually distant stars that move away from us at a speed greater than the ...
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1answer
385 views

Surely proper time expands like proper space?

The FRW metric at the origin $r=0$ is simply given by: $$ds^2 = -dt^2 + a^2(t)\ dr^2$$ Setting $dt=0$ gives us an element of proper distance $ds$ given by: $$ds = a(t)\ dr$$ Thus we get the well known ...