1
vote
0answers
49 views

Object that causes spacetime to expand?

Is there any thing that will cause spacetime to expand, so that particles are pushed away from them rather than pulled towards it. I know things such as black holes and planets causes dips and curve ...
4
votes
1answer
41 views

Vacuum energy: black hole evaporation and cosmology - a discrepancy?

Black hole evaporation is a result of calculating the expectation value of the stress-energy tensor of the state of the vacuum in certain spacetimes and then making a plausibility argument as to the ...
18
votes
2answers
393 views

Why does dark energy produce positive space-time curvature?

My understanding is that dark energy, or equivalently a positive cosmological constant, is accelerating the expansion of the universe and I have read that this gives empty space-time positive ...
2
votes
0answers
23 views

How do you extract cosmological constants like $k$ or $H_0$ from the fluctuations in the CMBR (WMAP)?

I am currently reading a book about Astrophysics and also tried to find some information about it on the web, but was not able to figure it out. That is why I ask you guys now. Any help is greatly ...
1
vote
1answer
58 views

Expanding Universe now and $\sim14$Gyr ago

All know that Inflation may solve some of the problems of the Standard Model of Cosmology and today we suppose that the field responsible for that is the inflaton. The properties of the potential ...
0
votes
0answers
36 views

Eddington-LeMaitre model

I'm trying to solve Friedmann equations for $k=1 $ & $p=0$ (matter). These are equations: $$3(\dot{R}^2 +1)/R^2 -λ =8πGε$$ $$(2R\ddot{R}+\dot{R}^2 +1)/R^2 -λ=0$$ If we use $(\dot{R}^2 +1) /R^2$ ...
1
vote
1answer
116 views

Metric expansion of space - past, present and future

Scientists say that out universe is expanding faster and faster. This was detected via the red-shift effect. Farther galaxies are moving from us faster than those that are more closely. But how can ...
0
votes
2answers
230 views

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 ...
-2
votes
1answer
109 views

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, ...
2
votes
1answer
90 views

Relation between $\Lambda$ and $\Omega_\Lambda$ in $\Lambda\mathrm{CDM}$

In minimal $\Lambda\mathrm{CDM}$, there is a parameter labeled $\Omega_\Lambda$, and current fits place it at around $\left( \Omega_\Lambda \sim 0.73\right)$. Meanwhile, $\Lambda$ enters the Einstein ...
2
votes
1answer
417 views

What is the geometry of our universe?

I have some questions about the notion of space in current cosmological theories. I have been trying to decipher a few papers written on this topic such as this one, but I keep running into some ...
4
votes
2answers
128 views

Cosmological triangle with PLANCK results

Is there an updated version of the cosmological triangle with recent PLANCK results included?
2
votes
1answer
257 views

Cosmological constant

I have always wondered about how cosmological constant is characterized. So since it is still a hypothesis you often read the “cosmological constant measured to be ….”. Shouldn't the statement read ...
-4
votes
1answer
656 views

Is antigravity the source of accelerating expansion (dark energy)?

Is antigravity the source of accelerating expansion(dark energy)? From the observation of 1998, we found that our universe has been continuing accelerating expansion, and the unknown cause for this ...
0
votes
2answers
224 views

What are the differences between dark energy and a cosmological constant?

My possibly mistaken understanding is that dark energy changes with time, whereas a cosmological constant is, well, constant. What about gravitational clumping? Detecting relative motion?
0
votes
2answers
117 views

Cosmological constant of standard model of cosmology and observational data

I am curious whether the current Lambda-CDM model of cosmology matches well with observational data, especially expansion of the universe. How well does Lambda-CDM defend its established status from ...
2
votes
1answer
112 views

extracting energy from cosmological expansion

This question is a more concrete reincarnation of an old question about energy conservation in GR. Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
1
vote
1answer
76 views

If space is being doubled, how fast is it doubling? [duplicate]

Possible Duplicate: How long does it take for expanding space to double in size According to the standard concordence model, I heard that it's likely that space is doubled after 11.4 ...
11
votes
2answers
283 views

How is causal patch complementarity compatible with behavior during inflation?

Causal patch complementarity is the conjecture that in de Sitter space with a positive cosmological constant, the states within the causal patch are sufficient to fully describe the universe with the ...
6
votes
1answer
71 views

Are there stable string theory vacua with non-minimal cosmological constant?

Naive reasoning suggests that a string theory vacuum with cosmological constant Lambda1 is always unstable as long as there is a string theory vacuum with cosmological constant Lambda2 < Lambda1 ...
2
votes
1answer
163 views

Does anisotropic expansion of the universe imply quintaessence?

a Recent paper establishes under solid grounds anisotropy in the expansion acceleration rate in the universe. My question is very simple: can this anomaly be explained entirely in terms of a ...
7
votes
3answers
1k views

Do the laws of physics evolve?

Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...
2
votes
2answers
640 views

What is meant by positive and negative gravity/energy/spactimecurvature?

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 ...
7
votes
1answer
645 views

On Flatness problem, Inflation etc

I have a couple of naive questions from the topic of the title. We know \begin{eqnarray} \Omega-1=\frac{k}{a^2H^2}-\frac{\Lambda}{3H^2} \end{eqnarray} Now I read that from the standard big bang ...
20
votes
3answers
760 views

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 ...