3
votes
0answers
38 views

Could the singularity that gave rise to our universe be past-eternal?

Are there any compelling models that include a past eternal singularity that ultimately gave rise to our universe? Does the "no-boundry" hypothesis that utilizes imaginary time have past eternal ...
7
votes
2answers
480 views

What is “Energy” of a vacuum in the context of quantum theory?

I'm not a physicist or cosmologist, so I hope I am asking the right question with the right words here. My question regards the word "energy" as it pertains to quantum vacuum states, and to concepts ...
2
votes
1answer
65 views

Friedmann Equations with varying G?

If Newton's constant $G$ actually varies with cosmological time $t$ would a suitably modified form of the Einstein field equations: $$G_{\mu \nu} + \Lambda g_{\mu \nu} = \frac{8 \pi G(t)}{c^4} T_{\mu ...
1
vote
1answer
126 views

Is the cosmological redshift caused by the Planck mass increasing?

The standard explanation for the cosmological redshift is that photons emitted from far away galaxies have their wavelengths lengthened as they travel through the expanding Universe. But perhaps the ...
7
votes
1answer
221 views

The BICEP2 data are evidence of gravitational waves and of inflation. Are they also the first observation that requires quantum gravity?

It strikes me that the recent announcement of data from BICEP2 contains two really Big Deals: the first evidence of gravitational waves the first evidence of inflation. Is there also a third? ...
6
votes
1answer
388 views

Is Poincare recurrence relevant to our universe?

If the theory of everything indicates a singularity-free and finite universe, will Poincare recurrence be relevant to the universe? If so, is there any interesting physical consequence, e.g. in ...
2
votes
0answers
53 views

Stringy corrections to Friedmann equation

Does anyone know a reference or a paper which discusses string theory correction to Friedmann equations?
2
votes
2answers
198 views

Is inflation deterministic?

In some theories inflation is supposed to be able to turn quantum fluctuations into macroscopic inhomogeneities. I don't understand how an isolated system such as the universe can undergo such a ...
4
votes
1answer
512 views

Is spacetime an illusion?

In consistent histories, for gauge theories, can the projection operators used in the chains be not gauge invariant? In quantum gravity, for a projection operator to be gauge invariant means it has ...
2
votes
1answer
485 views

Dirac Equation in General Relativity

Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation: ...
4
votes
3answers
357 views

Question on inflation

I have two particular questions regarding the inflationary scenario. They are: 1.) What is the physical origin of the inflaton field? 2.) Why has the potential of the inflation field its particular ...
3
votes
1answer
243 views

Question on inflation as a phase transition

I have just finished watching the following video http://www.youtube.com/watch?v=beQ9fZ0jVdE where Laughlin, Gross and some students discuss e.g. about inflation. The following question is risen: Is ...
2
votes
0answers
69 views

Wick rotation for FRW in quantum gravity

There is no timelike Killing vector for FRW cosmologies. In the path integral formalism, is it possible to Wick rotate for quantum cosmology in quantum gravity? If yes, how? If no, how does one work ...
1
vote
1answer
833 views

Branes Collision -> Big Bang

Imagine universe occurred when two parallel branes collided, Momentum of Branes converted to big bang kinetic energy after Collision. Thus, high-energy quanta are high-Vibrating strings. what ...
10
votes
7answers
2k views

What are cosmological “firewalls”?

Reading the funny title of this talk, Black Holes and Firewalls, just made me LOL because I have no idea what it is about but a lively imagination :-P (Sorry Raphael Bousso but the title is just too ...
5
votes
0answers
152 views

Implications of Unruh-inertia to theories of gravity

If it turns out to be true that the galaxy rotation curves can be explained away by Unruh modes that become greater than the Hubble scale at accelerations around $10^{-10} m/s^2$ as proposed in here, ...
14
votes
8answers
2k views

Reversing gravitational decoherence

[Update: Thanks, everyone, for the wonderful replies! I learned something extremely interesting and relevant (namely, the basic way decoherence works in QFT), even though it wasn't what I thought I ...
0
votes
1answer
68 views

In the Wheeler-de Witt formalism, do big bangs act as sources and future singularities as sinks?

In the Wheeler-De Witt formalism of quantum gravity, do big bangs act as sources and future singularities as sinks? A finite time ago, existence emerged from the big bang, and existence will fade in a ...
0
votes
1answer
79 views

Can anything come out from the big bang?

If any configuration of matter can fall into a black hole and hit the singularity, and ditto for the big crunch, and there is time reversal CPT invariance, does it mean anything can pop out of the ...
1
vote
1answer
68 views

Thermal gravitational radiation and its detection

To my poor knowledge on the topic, the gravitational waves that are most likely to be detected by LIGO or other experiments do not have thermal spectrum. But I'm not certain. I know that Hawking's ...
3
votes
2answers
240 views

Was Planck's constant $h$ the same when the Big Bang happened as it is today?

Was Planck's constant $h$ the same when the Big Bang happened as it is today? Planck's constant : $$h= 6.626068 × 10^{-34}\, m^2 kg / s,$$ $$E=n.h.\nu,$$ $$\epsilon=h.\nu$$
4
votes
1answer
220 views

Is the only diffeomorphism invariant anthropic principle the final anthropic principle?

Quantum gravity is a gauge theory with the gauge symmetry spacetime diffeomorphisms. Presumably the quantum state of our universe is invariant under spacetime diffeomorphisms, including timelike ...
2
votes
1answer
306 views

Does a complete theory of quantum gravity require anthropic post-selection?

Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least ...
4
votes
1answer
465 views

What are the theoretical explanations behind the “no-boundary condition” in cosmology?

In his book "The Grand Design" on page 167-173 Stephen Hawking explains how one can get rid of the so called "time zero issue", meaning by this the very special initial state needed for inflation to ...
11
votes
2answers
300 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 ...
2
votes
1answer
420 views

How is the expansion of space itself explained in LQG?

To explain the expansion of space I have often heard people saying that space is continuously created. This picture is usually applied to cosmological scales but I`m nevertheless curious if some ...
2
votes
2answers
185 views

Can the implications of dark energy be used to bridge the gap between Quantum Mechanics and General Relativity?

Can the findings of the Physics Nobel Laureates of 2011, namely the overpowering existence of dark energy (vacuum energy) have any implications in the quest the combine Quantum Mechanics and General ...
4
votes
2answers
1k views

Physical laws prior the big bang (quantum fluctuations)

A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me. What I can't grasp yet is how a quantum fluctuation can even start ...
9
votes
3answers
399 views

Does the dimensionality of phase space go up as the universe expands?

Ever since Hubble, it is well known that the universe is expanding from a Big Bang. The size of the universe had gone up by many many orders of magnitude as space expanded. If the dimensionality of ...
3
votes
1answer
788 views

Mathematical justification of Hartle-Hawking “no boundary” proposal

In Hartle-Hawking "no boundary" proposal it is proposed that Riemannian spacetimes rather than Lorentzian dominated the path integral near the big bang. Moments after the big bang however spacetimes ...
4
votes
4answers
683 views

Question regarding the validity of the big bounce

I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows. 1) How one reconciles it with the fact that it is now ...
8
votes
1answer
325 views

Summing over topologies

Sometimes in quantum cosmology, when we are thinking about 'wave functions of the universe' we have in mind some sort of formal path integral, where we include not just the variations in the dynamical ...
2
votes
1answer
349 views

Possibly naive question about quantized space-time

I beg your pardon in advance if this question is naive. In Quantum Mechanics, discrete values of measurements occur only in relation to bound states. This is because of the general solution for the ...