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1

Ok, so I've finally got to the bottom of this. Basically, the confusion stems from what it means to say the universe is accelerating so let's clear that up. The Hubble parameter is defined to be $H(t)=\dot a(t) /a(t)$ where $a(t)$ is the scale factor. When we say that the universe's expansion is accelerating, we mean that $\ddot a$ is greater than zero. This ...


0

How would we go about testing whether or not the vacuum itself gravitates? Do we need to? General relativity is one of the best-tested theories we've got, see Clifford M Will's paper at http://arxiv.org/abs/1403.7377. And in the Foundation of The General Theory of Relativity Einstein did say this: "the energy of the gravitational field shall act ...


0

One that is particularly interesting: In a paper published in Physical Review X, physicists Michael Hall, Dirk-André Deckert, and Howard M. Wiseman have proposed a new view of quantum mechanics that may be testable in a way that could prove that it alone is the correct interpretation. They call it the Many Interacting Worlds approach to quantum mechanics. ...


1

This seems to be still very much a mattter of opinion, based on the excerpt below from Space.com, which sums it up nicely and which you may already have read. Lots of questions currently with not enough data to resolve, esp. with regard to dark matter and questions regarding the first stars. Fingers crossed that LIGO and the JWT give us a clue in the ...


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Maybe your confusion comes from thinking that oscillations must be perpendicular to the direction of propagation, but it doesn't. The fluid does oscillate, just as a sound wave in air oscillates. Hence the name baryonic acoustic oscillations (BAOs). Particles are compressed, and this compression moves away with the speed of sound (which is something like ...


0

I was watching a Physics TV show, When someone called Alex Filippenko said that when there was the Big Bang, the Space extended at a speed faster than speed of light. He said that it wasn't against the theory of relativity because space isn't a particle and can go faster than speed of light. As far as I know it's true. Imagine space is like a stress ...


0

Because there was no space around it, all matter has a speed barrier which is the speed of electromagnetic radiation in vacuum (and gravitons in case they exist) inside the space time fabric of our universe, the boundary of the universe doesn't have that limitation since there is nothing outside, at least that is the proposal to explain how the universe ...


0

Yes, if the extra curvature is due the energy density of some unusual stuff that we just haven't seen yet, then it is reasonable that the density would be different at different places and times in the universe. However, if experiment shows the extra curvature to be the same everywhere, then it may be a new physical constant ... and not due to the density ...


2

Both the isotropy and homogeneity are amenable to observational test. One can measure the peak brightness and redshifts for type Ia supernovae in different directions and at a range of redshifts. One can then see whether the same cosmological models (including $\Lambda$) are required or can consistently model all datasets, or whether there are angular ...


1

The Chronology Protection Conjecture is an entire bundle of rough theorems, counterexamples and conjectures. Hawking's original paper on the topic hinges on two main arguments : That compactly generated closed timelike curves (aka "a time machine", roughly) will violate the energy conditions. That a Cauchy horizon (the part of spacetime where the time ...


2

There have been plenty of studies on the connection of long-duration GRB rates and star formation (e.g., Robertson & Ellis 2011, Trenti, Perna & Tacchella 2013 and Wang 2014, all arXiv links). The relation comes from observations of star formation history, $\dot{\rho}_*(z)$, and the number of gamma-ray bursts, $dN/dz$, with $z$ being redshift in both ...


-2

Do gamma ray bursts play a role in cosmic evolution? I don't really know the answer to that. But I imagine black hole jets in general play some kind of role. Have you ever read any articles like Black Hole Creates a Galaxy? "The astronomers think the black hole is powering star formation in the nearby galaxy by spraying its jets of high-energy ...


1

There are probably duplicates/variants of this question on this site. Expressing the problem in mass terms seems a bit odd to me though, as it's been put in energy terms every other time I have seen it asked, unless I have misunderstood your question. Here is an answer (to an identically worded question) based on the link: Predicted Mass of Quantum Vacuum ...


0

In principle, the number density of photons include all photons, both of cosmic origin (e.g. the CMB) and of astrophysical origin (starlight, gamma-rays from GRBs, radio waves from QSOs, etc.). However, most of these photons are CMB photons, as seen from this figure (from Lacasa 2014): The second largest contribution to the photon density are the cosmic ...


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I think in general there are a wide class of models that should pass cosmological and astrophysical constraints. The Living Review on this is a good place to look. In section 14 they explain the basic constraints on allowed types of $f(R)$ models. It seems like they amount to $f_{,R} > 0$ and $f_{,RR}>0$ when $R>R_0$, where $R_0$ is the scalar ...


0

If all of physics is reversible like some say, then the the increasing entropy of the Universe is causing us to burn fossil fuels as much as we are increasing the entropy.


3

In principle, no, you cannot make a Dyson sphere which is indistinguishable from the CMB. The reason is fairly simple. Let's start with a blackbody DS which encloses nothing at all, and is so far from any nearby stars that no noticeable radiation reaches it. Since it is surrounded by CMB with an effective temperature of 2.75 K, it will reach an equilibrium ...


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I don't think so Dirk. GRBs don't happen for nothing. See this 2001 paper by Friedwardt Winterberg. I think it's essentially correct. Why? Take a look at Einstein saying light curves because the speed of light is spatially variable. Also see Shapiro, and this Baez article: Einstein talked about the speed of light changing in his new theory. In the ...


1

If you built the sphere, at the optimum radial distance, then insulated the exterior as much as possible, would the gravitational redshift provided by the black hole not act to sort out your problem for you, if you want to dissipate it, as far as co-ordinate observers at any reasonable distance were concerned? Would accretion discs and the massive gravity ...


2

there are many experiments that put different contraints on the neutrino masses. Here is a good collection from the particle data group. To summarize: There are lots of experiments that put upper bounds on the neutron mass. The PDG groups estimate is that $\nu_e < 2eV$, $\nu_{\mu} < 0.19eV$, $\nu_{\tau} < 18MeV$. All with a confidence level of ...


0

Like Jimself, I know I've seen this somewhere (and will try to dig it up), but in the meantime I'll give you the answer off the top of my head. Can't guarantee this is entirely correct until I do dig some things up, but some parts are true (confident in the flat Universe part!). As long as you're interested in our Universe, your idea will actually work, ...


4

Okay, let's start with the basics. The Big Bang was not like an explosion in space from which spewed all matter in the universe. The Big Bang was a moment in time. We have this thing called a spacetime metric. I won't bore you with the details, but essentially it is the equation we use to describe all of the geometry in the universe. It includes all the ...


7

If you're asking whether there is any chance we'll be able to do this in the forseeable future then answer is no. If, however, you're asking whether general relativity allows constructions like this then the answer is yes. Your question refers to a brane which would exist inside a bulk of our own design, and I don't know how literally you meant this but it ...


1

The relevant part of the book is the section titled Motion through Spacetime in chapter 2. I'll copy the paragraph, but it's a bit long so feel free to skip over it: Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed—that of light. This is a strange idea; we are used to the notion that objects ...


0

Everywhere in the observerable universe is affected by gravity and since you can't escape it, that means it is always pulling on you. So every objecct in every place will be moving relative to everything else, so an absolute state of rest is impossible. If you could edit your post to quote the part of the book where he says "stationary", you will probably ...


0

The error is to imagine that the incredibly dense ball of matter was located in a larger universe. At that era, the incredibly dense ball filled the whole universe. It was, to a good approximation, already homogeneous and isotropic. If you believe in inflation, it made things even more so, but it wasn't a small blob in a large universe.


1

Atoms themselves didn't form until well after the Big Bang. The Big Bang Nucleosynthesis (BBN) is when most nuclei formed and that happened somewhere between 10 seconds and 20 minutes after the Big Bang (that's a long time relative to how quickly everything was happening back then). That would be when ions formed, neutral atoms didn't make an appearance ...


3

My intuition is that creating H gas in the lab is very hard (as opposed to H$_2$ gas). Not at all; any sufficiently hot hydrogen plasma will have a greater abundance of H than of H$_2$. To see why this is so, it is sufficient to consider the energies of the molecular bond relative to the ionization energy. The energy of the bond in a hydrogen molecule ...


1

Nobody knows. There are multiple explanations for dark energy that haven't been eliminated. One of the explanations that hasn't been eliminated is zero point energy: http://arxiv.org/abs/1205.3365. Another possible explanation is that the alleged expansion is actually a result of neglecting the effects of inhomogeneities on averaging: ...


0

Where does the energy go when light is redshifted? It doesn't go anywhere. Let's say we're motionless with respect to some source which is emitting a stream of photons. We agree that the photons have some energy E=hf. Now let's say I push you such that you're moving away from the source. You now claim that the photons are redshifted, but those photons ...


1

Nobody ever said that different observers have to agree on the energy of a photon (or anything else). The invariant quantity is energy minus momentum (i.e. rest mass), which is equal to zero whether the photon is red or green. (Edited to add: I see now that userLTK already said as much in a comment.)


0

Last I heard there was no direct experimental evidence to pinpoint the onset of the accelerated expansion. Evidence for this will probably come from very precise measurements of the Sn1a luminosities, but at the moment the data isn't precise enough. However the indirect evidence is compelling. We have established that general relativity gives a good ...


1

The hubble relation is: $$v = H d$$ where $v$ is the velocity of the galaxy relative to the Milky way, and $d$ is the distance of the galaxy relative to the milky way. The velocity is measured using redshift. The distance is measured through a complicated series of standard candles, along with the relationship $I = \frac{I_{0}}{4\pi r^{2}}$. If you ...


2

People are usually more interested in the reverse process of production, that is the annihilation of dark matter particles. This is simply because it may be easier to see the products of annihilation (which might produce photons as a by-product) than to notice a small amount of ordinary matter that has "disappeared" to produce dark matter. And finding ...


0

What Scott's saying Okay, so the basic assumption I can phrase as this: The universe is finite and has an end. During The End, cosmic inflation does not rip every particle apart into its own universe, and no cyclic model of cosmology predominates: The End of the universe is instead one big "soup" of particles in thermal equilibrium, all having some low ...


3

Q: When we write that, do we suppose a collisionless or collisional nature of the fluids? A: It's the energy-momentum tensor for a perfect fluid Chapter 2.26 Q: If this description corresponds to collisional fluids, why cosmological simulations are N-body simulations (collisionless) and are not simply based on hydrodynamics? A: Cosmological simulations are ...


3

I would say the sign of the cosmological constant would certainly play a factor in determining singularity behaviour of the universe. This can be seen from Raychaudhuri’s equation, which is precisely obtained from Einstein’s field equations, and is given by: $$\dot{\theta} + \frac{1}{3} \theta^2 + \sigma_{uv}\sigma^{uv} - \omega_{uv} \omega^{uv} + ...


-2

Singularities are very likely impossible to create in the real universe. In other words, as a singularity gets close to forming, the incoming random GR waves and other energy will rip the formation apart, keeping it in a state of almost singularity. As an example, all Black Holes spin in the real world. The size of the singularity in a spinning Kerr ...


1

A singularity involves an infinite amount of negative potential energy in a localized volume. A nonzero cosmological constant would only yield a finite amount of positive energy in a localized volume. So the cosmological constant might slow down the rate of singularity production, but it won't stop it.


0

However, if that is the case, than everything would technically pull on everything, right? No. A gravitational field is a place where space is "neither homogeneous nor isotropic". You can See Einstein talking about that here. And the FLRW metric "starts with the assumption of homogeneity and isotropy of space". I'm confident that this is correct ...


-2

I think, the physical signifcance of the Planck force comes fromthe formula: looks like it's the force needed to accelerate the Planck mass to the speed of light in the Planck time right?


0

I describe how to calculate the Hubble parameter in How does the Hubble parameter change with the age of the universe?. You should have a quick read through this as it's relevant to the rest of your question. We know the universe is expanding. We describe its size by a parameter called the scale factor, $a$. The rate of expansion is the rate of change of ...


3

This is a very broad question but I'll try and answer it. I'll not include any mathematics in my answer. If you would like me to get into technical details then let me know and I'll edit my answer. The idea of multiverses has found resonance in multiple places in theoretical physics but I think Brian Greene's categorization of multiverses is perhaps the must ...


0

I am attempting answering my question based on one fact and one assumption (actually there are one more fact/ observation - Redshift): The fact is: inertial frame. which, to a good approximation, earth itself (ignore its rotation, irrelevant to our discussion) is an inertial frame. The assumption is: Earth is no special place. Hence the logical ...


0

Dear Carson: To understand why we see today the CMB radiation, after 13.7 billion years after the BB we do not need to know GR at all. Let's use a story to simply illustrate the problem, and get the solution. Suppose you awake in the middle of your street and you find yourself sorrounded by a terrible heavy and dense fog, with a yellow glow all over. So ...


0

General Relativity is about describing (the dynamics of) a curved spacetime. So you need a collection of events, and a metric that tells you the interval between nearby events. That's it. You can a stress energy source term too. But anything else is either unphysical or a straight up bias brought into a theory for no reason. There are some easily ...



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