# Tagged Questions

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### The cosmological constant as a Lagrange multiplier?

The cosmological constant $\Lambda$ can be introduced into the gravitational action like this : S = \frac{1}{2 \kappa} \int_{\Omega} (R - 2 \Lambda) \sqrt{-g} \; d^4 x + \text{matter ...
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### Entropy of the cosmological constant and the laws of thermodynamics?

Convention The convention being used is: $A_{C} =$ The classical variable Premise Consider the following toy-model universe: A universe with a positive cosmological constant. Basic ...
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### Cosmological Constant on the LHS of Einstein's Field Equation

The cosmological constant seems to be normally described as an energy (repulsive force, Dark Energy) of Space-Time. I was just wondering, if we were to interpret the cosmological constant as being ...
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### Connection between the cosmological constant $\Lambda$ and the cutoff scale $\Lambda$

I'm trying to understand the connection between the $\Lambda$ from cosmology and the $\Lambda$ from QFT. Cosmology: The cosmological constant enters the Einstein equations. In the special case of the ...
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### Is it possible to directly test whether of not the vacuum gravitates?

According to GR, all sources of stress-energy (e.g. everything on the $T_{\mu\nu}$ side of the EFE) should gravitate (e.g. affect the curvature/$G_{\mu\nu}$ side of the EFE). We observe the expansion ...
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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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### 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 ...
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### Leonard Susskind said that the small value of the cosmological constant could mean that the universe is much bigger than we thought. Why?

He said that near the beginning of this Youtube video that it could mean that the universe is much bigger than he had thought that it was, and that the cosmological constant wasn't even a constant. I ...
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### Locality, unitarity & vacuum energy

I've read in a set of lecture notes that the requirement of locality and unitarity in QFT imply that the vacuum must have a non-zero energy associated with it (http://arxiv.org/abs/1502.05296 , top of ...
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### Why is Weinberg's prediction of omega sub lambda ($\Omega_{\Lambda}$) other than sophisiticated tautology?

I am not a physicist, but I do have a background in engineering (EE and computer science) and have followed physics and cosmology on a primarily conceptual basis for many years. I have looked at ...
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### Cosmological constant (positive, dS) as SUSY/supertring issue

The "almost" de Sitter expansion of current cosmological model implies a puzzle for supersymmetry (SUSY) and also for string theory. A positive value of the cosmological constant implies a non-null ...
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### A question on the Bousso-Polchinski paper

In this famous paper by Bousso and Polchinski, Quantization of Four-form Fluxes and Dynamical Neutralization of the Cosmological Constant an example in M-theory compactification is given in section ...
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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 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 ...
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### What prediction led to the vacuum catastrophe?

The disagreement between predicted and measured energy density of the vacuum is one of the great unsolved problems of science. According to this book the predicted energy density was obtained as ...
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### The fate of Poincaré recurrence with the Big Rip

Recently, there has been a lot of talk in the media about the "Big Rip". It most certainly resulted from the paper by Marcelo M. Disconzi and Thomas W. Kephart where they have figured out a ...
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### How to find de Sitter and almost de Sitter solutions in (super)string theory

From Cosmology, we have learned that we live in an almost de Sitter (positively accelerated!) Universe. It seems that dS space solutions in superstring/theory are problematic and there are some no-go ...
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### What are the zero point energy densities of the individual quantum fields?

I'm reading through "General Relativity - An Introduction for Physicists", by Hobson, Efstathiou, and Lasenby, and I have a question regarding one of the statements related to the cosmological ...
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### 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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### Has unnaturalness motivated new ideas in physics?

The cosmological constant problem arises because the natural scale of the cosmological constant is $10^{120}$ times larger than what we observe. This implies that the dimensionless constant is much ...
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 $(\dot{R}^2 +1) /R^2$ ...