# Tagged Questions

Dark energy is the unknown form of energy that drives the acceleration of the universe's expansion.

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### How does the inclusion of vacuum energy modify Newtonian gravity equation of motion?

The equation of motion (in the center of mass frame) due only to gravitational forces between two point masses is: $$\frac{d^2r}{dt^2} = -\frac{GM}{r^2}$$ How does the equation get modified when a ...
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### Is it possible for dark energy density be decreasing? [on hold]

I have an equation relating dark energy density and temperature, actually they have a linear relationship, my question is how to choose the temperature? If I choose the temperature of universe, the ...
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### Dark energy and magnetic field [closed]

I have learned that a photon creates it's own magnetic field. Arno Penzias & Robert Wilson discovered the background radiation from the Big Bang. Could this also be the time of the creation of ...
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### What is the difference between the Big Bang Model and the Λ-CDM Model?

If I'm going to write about "The Consensus Model of Cosmology" should I include Big Bang Model or should I go with just the $\Lambda$-CDM Model?
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### Could dark energy and Hawking radiation be the same thing? [closed]

Specifically, could black holes effectively emit some kind of negative mass or other form of dark energy, so that the mass of the black hole itself would actually increase rather than decrease ...
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### Exponentially accelerating universe, dark energy, the universe's temperature and event horizons.. (sorry in advance!)

Ok.. assume that space-time is accelerating away from itself with a>0 and.. jerk - j⃗ (t)>0 (sorry - don't know how to write "a dot"! :s ).. If this continues on without the rate of acceleration ever ...
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### How do we know that the galaxies the farthest away from us are actually speeding up?

What i mean is Hubble proved that the galaxies the farthest away from us were moving away faster than the galaxies closer to us. I have a basic understanding of red shift. I've always wondered (and ...
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### Does the Casimir effect give the correct value for Dark Energy?

My understanding is that the Casimir Effect is caused by vacuum energy. Quantum mechanics (QED) predicts vacuum energy, but gets the value grossly wrong, by a factor of $10^{120}$. On the other hand, ...
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### Dark matter and dark energy references

I've been looking for questions about dark matter, and I've read some very interesting answers. However, I desire too look into it deeply. This is not actually a question. I'm asking the community ...
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### What is dark matter? Also what is dark energy? I am not duplicating it put just want to write as an article for helping others [duplicate]

Today one of the greatest questions coming up is what is dark matter. also what is dark energy. we don't know much about it still!!! I have tried to bring some light on this question. To prevent ...
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### Dark energy and virtual particles

If space time is accelerating away from itself everywhere due to dark energy, could this acceleration be the cause of virtual particles, essentially Unruh radiation?
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### Rate of change of temperature of the Universe

Temperature of the whole universe is about 2.7K base on the wmap satellite. I know that temperature has to decrease to time and I have difficult time finding references for rate of change of ...
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### Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson? [closed]

Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson and all kind of these like stuffs?
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### Where did all the photon energy go?

From the wikipedia page on dark energy, in reference to the total mass-energy of the universe: The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, ...
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### Why doesn't the cosmological constant cause black holes to form?

The cosmological constant acts as matter with positive energy but negative pressure. Then a region $R$ should have energy $E\sim \Lambda r^3$. On the other hand, we know that the energy for a black ...
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### Are modified theories of gravity credible?

I'm a statistician with a little training in physics and would just like to know the general consensus on a few things. I'm reading a book by John Moffat which basically tries to state how GR makes ...
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### G4v Gravity Theory: Why does this get rid of Dark Energy?

Earlier this year, Carver Mead of CalTech published a paper which seems to be garnering a lot of attention: http://arxiv.org/abs/1503.04866 http://www.npl.washington.edu/AV/altvw180.html ...
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### What is the amount of dark energy in the Universe, in joules?

I'm toying with an idea, and need absolute amounts of dark energy. I'd also like the same for dark matter. If these values are not estimated, why not?
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### Could the universe really be expanding at a constant rate? [closed]

I was just thinking, sorry if this idea is idiotic, but since we know galaxies move away from each other at an accelerating rate, could this be due to a weakening gravitational force between galaxies? ...
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### Was the matter-energy content of our universe always distributed in the same ratios?

Currently, Dark energy (68.3%) and Dark matter (26.8%) together constitute about 95.1% total matter-energy content of the universe while only 4.9% is ordinary baryonic matter. Was this always the ...
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### 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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### Could a perfect vacuum exist at the boundary of the universe?

Space is not truly a vacuum because of our observation of quantum foam, but if causality does not exist outside our universe, could that mean that quantum foam cannot exist outside our observable ...
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### What is meant by positive and negative gravity/energy/spacetime-curvature?

I have recently come across some cosmological assertions (based on empirical data) about the universe being self contained in the sense that it is entirely capable of coming into existence from a ...
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### How come the cosmological constant supports the idea of dark energy existing? [duplicate]

According to an article I've read recently, the cosmological constant was used to support the theory of dark energy. But the cosmological constant already defines an inclination in the essence of ...
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### How to determine the size of other galaxy's/The dark matter problem

From what i understand there are two methods (well as far as i know) to determine how much mass there is in a distant galaxy. First, Luminosity. By measuring the amount of light produced from a ...
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### What are the alternative theories of dark energy? ($w \neq -1$)

There has been a lot of related questions about dark energy around here but these are usually 2-4 years old and the closest question to mine hasn't really been answered, so I am going to proceed. ...
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### Isn't the accelerating expansion of the universe intuitive?

I have a question about accelerating expansion of the universe. My understanding is that Hubble said the farther our we look into the universe, the higher the redshift, therefore the faster things ...
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### Doesn't the presence of dark energy make the flatness problem less important?

If the cosmological constant (aka dark energy / vacuum energy) were zero, then an Omega greater than 1 would mean both positive curvature and a recollapse while an Omega less than 1 would mean both ...
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### How much light is there in space and how heavy is it?

Our night sky is filled with stars. On a dark night a significant fraction of the sky is light. This light, we are told, has been in transit for many millions of years. There must therefore be quite a ...
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### gravity cancelling dark energy

Say there were 2 objects with certain masses (e.g. $m_1$ and $m_2$). If they were close together gravity would attract the 2 objects. If they were a large distance apart the expansion of the universe ...
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### Are there experiments taking place right now that might show evidence for or falsify dark energy or dark matter?

What experiments at quantum or cosmic level might provide the strongest supporting evidence for or, conversely, potential for falsifying, the existence of dark matter or dark energy?
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### Is it possible for the universe to be already developed when the dark energy appeared and expansion started?

The main piece of evidence that supports Big Bang Theory is basically evidence that the expansion of the universe started long time ago (not necessarily from the begining of the universe). Is it ...
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### Relation of General Relativity to Dark Matter and Dark Energy

I was reading an elementary book on dark matter (in fact, a historical perspective) and there were mentioned how the scientific community react to the idea of dark matter proposed as a solution to ...
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### Question of the scale factor $a(t)$ for the dark-energy, $\Lambda$, epoch

Just a quick question. I find a lot that the scale-factor $a(t)$ goes as $a(t)\propto{\exp(Ht)}$ in the $\Lambda$ dominated model, however, what I am specifically looking for is the exact scale factor ...
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### Dark Energy Density Paramater For Unlimited Expansion

Suppose that the universe is dominated by matter and dark energy. Let $\Omega_m$ and $\Omega_\Lambda$ denote the respective relative density parameters. Given $\Omega_m$, what does $\Omega_\Lambda$ ...
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### Matter and dark energy

So dark energy is the biggest thing in the universe, it keeps accelerating the expansion. But it's a constant so at some point there was a matter - dark energy equivalence. My question is how can you ...
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### Could black holes be driving the expanding universe (dark energy)?

I would like to start be saying I'm no Physicist. Anyway, my question is: Could black holes be driving the expanding universe (dark energy)? When I think of the image that the effect of massive ...
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### Repulsive forces at large distances

I'm currently writing a 3D $n$-body simulator (similar to what you see here to simulate the evolution of a system of n particles under the force of gravity. It works fine for "smaller" scale ...
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### Does matter shrink due to energy radiation and does it have anything to do with dark matter and dark energy?

Is matter shrinking? Is shrinking the cause of concentrated dark matter that makes up matter moving from concentration to unconcentration. Does dark energy and expansion of the universe come from this ...
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### Gravitational constant, $G$, What if it is not Constant?

The question is on title; What if Gravitational Constant, $G$, is not constant? Let's skip all the local consequences if it would suddenly change, like asked here; Can gravitational constant be ...
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### Why is the universe expanding while its components actually attract each other?

First of all, what's the difference between gravity & gravitation? Anyway that's not the actual question. If every matter attracts each other toward itself, then wasn't the universe supposed to be ...
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### Does dark energy work like gravity, but the opposite way?

If a body has more mass gravity will exert a greater force on it. Does that apply also to dark energy? In other words, if a body has more mass, will it be affected more by dark energy? (that is, will ...
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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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### Why is the expansion of universe continuously increasing? [closed]

If our universe is expanding and the concept of the multiverse is true, then why is the rate of the expansion of the universe increasing? Surely it must decrease?
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### Can Gravity be a repulsive 'push' from Dark Energy, rather than an attractive 'pull' from Matter?

Gravity makes things that are more dense fall. If we extrapolate this fact through the entire Universe, we can imagine Ordinary Matter falling more than Dark Matter, and Dark Matter falling more than ...
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### How do radio signals contribute to gravity?

First, the inspiration for this question: I just read that it takes one hour to send a picture from the New Horizons space probe, to Earth. It also takes around 5 hours for that picture to reach ...
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### Expansion of the Universe: Conversion of gravitational potential energy to kinetic energy?

Suppose there is an object floating in space which over time begins to fall toward the source of a gravitational field. As it falls, its motion happens to be such that it gets locked in orbit around ...
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### Could the acceleration of universe expansion be caused by gravity itself?

Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), ...