We’re rewarding the question askers & reputations are being recalculated! Read more.

Questions tagged [cosmological-constant]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
68
votes
6answers
12k views

Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?

In Newtonian gravity, an infinite volume filled with a uniform distribution of mass would be in perfect equilibrium. At every point, the gravitational forces contributed by masses in one direction ...
48
votes
3answers
3k 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 ...
27
votes
6answers
7k 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 ...
27
votes
1answer
7k views

Why are anti-de Sitter spaces so interesting when we believe the universe is expansionary?

Perhaps this is a naive question, but in my recent (admittedly limited) readings about AdS spaces, I keep wondering why they seem to be such a hotbed for theoretical research (AdS/CFT correspondence, ...
26
votes
2answers
3k 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 ...
17
votes
1answer
2k views

Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy?

Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy? The accelerated expansion only tells us that the Einstein field equation must contain a ...
16
votes
1answer
671 views

The cosmological constant as a Lagrange multiplier?

The cosmological constant $\Lambda$ can be introduced into the gravitational action like this : \begin{equation} S = \frac{1}{2 \kappa} \int_{\Omega} (R - 2 \Lambda) \sqrt{-g} \; d^4 x + \text{matter ...
15
votes
2answers
3k views

Is the Cosmological Constant locally zero?

On earth and in our solar system we do not notice any effects of a non-zero cosmological constant. The accelerating expansion of the universe was only detected by observing the most distant supernovae....
15
votes
2answers
7k views

Lorentz invariance of the Minkowski metric

As far as I understand, one requires that in order for the scalar product between two vectors to be invariant under Lorentz transformations $x^{\mu}\rightarrow x^{\mu^{'}}=\Lambda^{\mu^{'}}_{\,\,\...
14
votes
4answers
1k views

Why didn't Newton have a cosmological constant

Einstein initially added the Cosmological Constant because (if I get this right) it seemed to him that the universe should be static. I agree that back then this would have been an obvious assumption. ...
13
votes
1answer
2k 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 (SBB) ...
13
votes
2answers
519 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 ...
12
votes
7answers
2k views

Strong empirical falsification of quantum mechanics based on vacuum energy density

It is well known that the observed energy density of the vacuum is many orders of magnitude less than the value calculated by quantum field theory. Published values range between 60 and 120 orders of ...
10
votes
1answer
440 views

Is broken supersymmetry compatible with a small cosmological constant?

I understand that we can find the energy of a bosonic field in its vacuum state via $E_{vac}^{(B)} = \sum_{\vec{k},s} \frac{1}{2}\hbar\omega_{\vec{k},s}^{(B)}$ and a fermionic one similarly, $E_{...
9
votes
5answers
3k views

Negative pressure, tension, and energy conditions

We have lots of common everyday experience with positive pressure, the canonical example is a gas. But other examples of positive pressure are easy to imagine: for instance, a solid that gets ...
8
votes
3answers
2k views

Vacuum and repulsive gravity

How can one show from General Relativity that gravity is attractive force, and under which conditions it becomes repulsive, also why positive energy vacuum drives repulsive gravity?
8
votes
1answer
656 views

Naturalness arguments and dimensional regularization?

How do issues of naturalness arise when regularizing QFT using dimensional regularization? I can only recall ever seeing naturalness arguments (hierarchy problem, cosmological constant problem, etc.) ...
8
votes
1answer
195 views

Only vacuum is possible in the large $D$ limit of General Relativity?

The Einstein equations with a cosmological constant $\Lambda$ read as: $R_{{\mu}{\nu}}-\dfrac{1}{2}Rg_{{\mu}{\nu}} + \Lambda g_{{\mu}{\nu}} =8\pi T_{{\mu}{\nu}}$ Therefore, $R-\dfrac{D}{2}R+D\...
8
votes
3answers
1k views

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 http://...
7
votes
1answer
216 views

Can a closed universe become open?

My feeling is that no a closed universe can't become open. Under normal evolution according to the Friedman equation the curvature would be more exagerated as time goes on (i.e. a a closed universe ...
7
votes
2answers
244 views

Lemaître's hesitating universes

I'm looking for references on Lemaître's "hesitating" universe models, defined by a long period of stagnation of the cosmological scale factor. These models are using a cosmological constant $\Lambda ...
7
votes
4answers
1k views

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, and ...
6
votes
3answers
301 views

Does string theory accommodate effects of dark energy?

String theory is arguably, among best candidates for a Theory of Everything. So, because every TOE is an attempt to bind together the big and very small effects and things, a TOE must be able to ...
6
votes
1answer
156 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 ...
6
votes
2answers
383 views

How can a cosmological constant be a candidate for dark energy if the universe is flat?

How can a cosmological constant be a candidate for dark energy if the universe is flat? Doesn't a cosmological constant in EFE give rise to positive/negative or no curvature to the universe? I mean, ...
6
votes
2answers
367 views

Would cosmological redshift be present in the following situation?

I'm trying to understand if cosmological redshift is just a secondary form of doppler redshift, or something else entirely. Suppose the two galaxies in the picture are receding from each other, but ...
6
votes
1answer
543 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 ...
6
votes
1answer
116 views

Interplay between the cosmological constant and “microscopic” properties of string vacua

As far as I understand, string phenomenology is usually concerned with compactifications of string theory, M-theory or F-theory in which the uncompactified dimensions form a 4-dimensional Minkowski ...
6
votes
1answer
132 views

What does the Friedmann Equation imply for the cosmological constant?

The Friedmann equation is given by $$(\frac{\dot{a}}{a})^2 = \frac{8 \pi G \rho}{3} - \frac{kc^2}{a^2} + \frac{\lambda c^2}{3}.$$ Considering that the left hand side is analagous to the kinetic energy ...
6
votes
1answer
195 views

Could dark energy make a large black hole less black?

Theoretically, if a black hole grew to a huge scale such that the effect of dark energy was large, could the black hole become 'normal' space again (i.e. no horizon or singularity)? What I'm trying ...
6
votes
1answer
524 views

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 ...
6
votes
1answer
144 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 ...
6
votes
0answers
115 views

Meaning of the simplest potential of quintessence models. Fields in denominator?

I am reading Sec. 1.12 of the Cosmology book by Weinberg. In this section he explains the very simple model of quintessence which attempts to provide a dynamical explanation of the smallness of the ...
6
votes
0answers
2k views

A Universal Upper Limit on Mass Within a Radius $R$?

Since the universe has a positive cosmological constant, there is an upper limit on the mass of the black holes as evident from the so-called Schwarzschild-de Sitter metric: $ds^2 = -f(r)dt^2 + \...
5
votes
1answer
1k views

Is there a relationship between the Cosmological constant and the Hubble constant?

I looked around to see if this precise question was asked before and it appears not to be. So is it just me or has anyone else noticed that, no matter what consistent set of units you use, $$ \...
5
votes
1answer
228 views

Fine Tuning of the Universe

I'm an A level student looking into the fine tuning of various constants. Physicists explain the extensive effects that would happen if these constants were to be changed/different and hence, how this ...
5
votes
2answers
130 views

What evidence is there for additional dark energy coming into existence when space increases?

As I understand since cosmological constant is a 'constant' - increasing the space must generate additional dark energy that fills that space This does sound counter-intuitive to me, but I am sure ...
5
votes
3answers
1k views

Vacuum energy and perpetual motion

The part of the Einstein equations of general relativity referred to vacuum energy, introduce a repulsive term in gravity. This means that as the space become bigger and bigger, vacuum part become ...
5
votes
1answer
186 views

Does string theory give any values for the predicted “leakage” of gravity into extra spatial dimensions?

Sorry if this is a duplicate. I have read that string theory has a good explanation for the "hierarchy" problem of the force strength disparity between the fundamental forces because as gravity ...
5
votes
1answer
87 views

What theories purport to describe “physical-level understanding” of dark energy?

What theories propose to close the gap (as suggested by http://arXiv.org/abs/1210.6008) between 'phenomenological behavior of dark energy' and 'physical-level understanding of dark energy' and to what ...
5
votes
1answer
625 views

de Sitter versus Minkowski QFT and cosmological constant

WMAP/Planck results confirm than we live in a de Sitter-like phase, i.e., a Universe with positive acceleration or positive cosmological constant! Therefore, I believe that a way to solve the ...
5
votes
1answer
146 views

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 ...
4
votes
3answers
1k views

Why isn't Anti de Sitter space taken seriously as a model of reality?

I asked a few questions here earlier regarding a physics model which could possibly point to Eternal Return, and was pointed towards Anti De Sitter space. Arguments in theories for Eternal Return ...
4
votes
4answers
1k 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?
4
votes
2answers
565 views

Can String Theory really fail to contain a de Sitter vacua?

I was reading a post earlier from Peter Woit's Not Even Wrong blog and came across the following reference to the paper "What if string theory has no de Sitter vacua?" by Ulf H. Danielsson, Thomas Van ...
4
votes
1answer
105 views

What is the analogous effect of cosmological redshift for particles with mass?

Due to the metric expansion of space, light emitted from earlier times in faraway parts of the universe and travels for a long time to reach us get progressively redshifted, decreasing the energy of ...
4
votes
1answer
68 views

“Cosmological constant problem” and dark energy

Simply stated: is the "cosmological constant problem" (the discrepancy of about 120 orders of magnitude between measured values of vacuum energy and the ones predicted by Quantum Field Theory) in any ...
4
votes
1answer
164 views

Why are only energy differences measurable?

In the framework of special relativity (SR), why is it that only energy differences are observable? Is it simply because, starting our with some action $$S=\int d^{4}x\,\mathcal{L}(x)$$ one can ...
4
votes
1answer
91 views

If non-zero cosmological constant interpreted as a repulsive field, what would be the properties of this field's quanta?

If non-zero cosmological constant interpreted as a repulsive field, what would be the properties of the excitation of such field, i.e. the particle which serves as the field's quantum? What would be ...
4
votes
2answers
325 views

Cosmological triangle with PLANCK results

Is there an updated version of the cosmological triangle with recent PLANCK results included?