# Tag Info

62

I'm not a quantum cosmologist, but I am an early-universe cosmologist, so I can give you my opinion after having read this paper. The article claims that Bohmian trajectories is a valid replacement for geodesics. This was claimed in the very beginning of the paper and not much is offered in the way of defense for this assumption. That's not to say that it's ...

26

While this work certainly investigates an interesting point, I think simply replacing geodesics in GR with similarly looking quantum trajectories does not solve the issues here. Finding the Friedmann equations while assuming large-scale homogeneity and isotropy is no surprise to me. There are a number of people working on so-called Big-Bounce Cosmologies. ...

8

There are a number of models for the universe over the years. The Big Bang as you show it in the figure has become the "standard model" for the creation of the observed universe as we know it because it fits observations, i.e. data, using known theories and behaviors from elementary particle theories. This model has been evolving as data are added in our ...

5

The top of the observable Universe it currently about 46 billion light years up. But the Earth is spherical, so which way is up changes depending where you are, and could be in any direction. Because the Universe is expanding (in the sense of metric expansion), and because the observable Universe grows with time (because light has time to travel further), ...

5

No, it's not possible. The other galaxies we see are to radically different. Additionally, if we are the surface volume of hyperspace, then the universe should be closed. Our best estimates and observations indicate it's flat. Let me address both of these in more detail. As for the other galaxies. First of all, there's the Andromeda galaxy. That is a galaxy ...

4

Fortunately, explosions tend to decay in intensity like $1/r^2$, so no known celestial events (e.g. supernovae) would really be able to do that. In any case, with these sorts of things, we could presumably see that such an event was "about to happen" and had simply not happened yet. There are actually some awesome doomsday cosmologies which are related to ...

4

General relativity was written to describe all behaviour of spacetime. And expanding universe is one small subset of the whole general class of spactimes in the phase space of general relativity. So yes, relativity does apply to an expanding universe. At least general relativity does.

4

The fluctuations in the CMB you refer to are over-densities and under-densities in the matter distribution of the early universe (among other things). An over-density is where a region of space has a more dense distribution of matter than the average value of the rest of space. Quite literally, the density is over the background value. If the mass ...

4

The fluctuations that are seen in the CMB are spatially too large to collapse into stars. They are the protoclusters and proto-superclusters of our universe. However, the amplitude of the CMB fluctuations is not large enough for them to gravitationally coagulate into the clusters and galaxies we see today at all. (Cold) dark matter is essential in this ...

4

With dark energy, the event horizon converges to a finite value so does it mean that it is constant over time? The event horizon will converge with the hubble radius in about 16 billion years: It will approach, but never reach, a fixed value of about 18 Gigalightyears. If we can observe a galaxy now, we can always observe it in future because the ...

4

A few things: 1) it is in principle unknowable what's happening outside the cosmological horizon. Because notions of total energy depend on boundary conditions (or conditions at infinity), there are several different possible scenarios for "the total energy of the universe", all of which are completely consistent with observation 2) Assuming that ...

4

There are several ways of answering this question. We may have additional information (not including a parallax!). For instance, if we know the surface temperature of the star and its gravity, both of which can be estimated directly from spectroscopy, then the type of star leads to a direct prediction of its absolute luminosity. This in turn leads to a ...

3

It seems that the answer to my question is that there's a difference between the pre-inflation universe "singularity" and that of other types of singularities like a black hole. It wasn't a singularity as describes a black hole, it was a point in time where the scale of the universe was zero. All that exists didn't occupy a single point in space in the way ...

3

The only error is in taking $\Omega_\mathrm{R} \approx 0$ to be exactly the same as $\Omega_\mathrm{R} = 0$. The radiation density is not exactly $0$ today; it's just small enough to no longer matter. However, as you go back in time ($a \to 0$), $\Omega_\mathrm{R}(a)$ grows faster than either $\Omega_\mathrm{M}(a)$ or $\Omega_\Lambda(a)$. Thus there is some ...

3

So: fusing hydrogen to helium yields 0.7% of the hydrogen mass as energy. The amount of hydrogen required per second if you have a 100% efficient energy capture process and need the power of the Sun is $3.8\times 10^{26}/(0.007c^2) = 6\times10^{11}$ kg/s. Provided you can figure out how to process this (and that seems much harder than building a Dyson ...

3

While a B-mode signal in the CMB would be a smoking gun for inflation, inflation is not even under pressure, even if BICEP2 only measured dust. Inflation explains a number of early universe puzzles neatly. Most importantly it explains why we find an extremely homogeneous CMB. Furthermore, without inflation it is hard to explain that the perturbations on the ...

3

Time is affected noticeably by gravity only in strong gravitational fields, i.e. in the vicinity of compact objects, so time doesn't run differently in voids from average regions. Dark matter (DM) and energy (DE), on the other hand, can to some extend be unveiled by studying voids. The morphology of the voids is affected by the nature of the DE, so ...

2

You would be very interested in one of the recent Kepler discoveries - Kepler 444. The star is estimated to be 11.2 billion years old (using asteroseismology) and is surrounded by a number of rocky exoplanets. These planets are all too close to their parent K-dwarf star to be in the habitable zone, but there is no reason there couldn't be planets further ...

2

Sometimes the word universe is just used colloquially and can just refer to everything on some side of a horizon (an event horizon, a causality horizon, etc.) But when used precisely, I'm sure different definitions are used in different fields. For instance, in mathematical general relativity, you assume that your universe is a connected four dimensional ...

2

If the question is asking whether there is a definition that encapsulates our universe, then I believe the answer is No. This is because encapsulating a "space" into a formal system requires defining bounds. However, we don't know the bounds of our own universe--let alone what bounds might be possible for any universe. We can only describe what we can ...

2

Lyman Alpha absorption systems The Lyman-$\alpha$ Forest and Gunn-Peterson troughs are two extremes on the scale of absorption features that are left by neutral hydrogen in intergalactic space. When ultraviolet light from a background source, typically a Quasar or a young, strongly star forming galaxy, travels through intergalactic space, it is ...

2

This is a difficult question for many reasons. One reason is likely because most of the introductory thermodynamics textbook problems that we are familiar with from childhood do not involve gravity. To illustrate this difficulty with gravity consider, for example, this snippet from an article in the New York Times Review of Books by physicist/mathematician ...

2

Short answer: yes. But what do the original "straight lines" mean now? they cannot be defined in a nice way in the new metric, because the natural geometric entities now are geodesics, and a quadrilateral of four right angles does not exist (apart from possible special choice of corners). You must define your volume in a correct way (see below) Long answer: ...

2

Dark energy is not negative energy. It causes a repulsion because of its unusual equation of state, which causes it to behave as if it has a negative pressure. There is some discussion of this in the answers to Have negative pressures any physical meaning? and possibly also 'Negative pressure' counteracting gravity?. When general relativists talk ...

2

Long story short: conservation of energy only holds locally where you can assume a static spacetime. On large scales the expansion of the universe gets relevant, so energy is said not to be conserved universally since the amount of dark energy per volume stays the same while the volume increases, see Sean Carroll's article, from which I quote: The famous ...

2

Whether energy is or isn't conserved in an expanding universe is a somewhat vexed issue. On the one hand you have an experienced physicist claiming that energy is conserved, and on the other hand you have an experienced physicist claiming that energy is not conserved. The problem is that accounting for energy in general relativity is a complicated business. ...

2

Quoting Sean Carrol's article linked by Симон Тыран, which makes the case for energy not being conserved: Having said all that, it would be irresponsible of me not to mention that plenty of experts in cosmology or GR would not put it in these terms. We all agree on the science; there are just divergent views on what words to attach to the science. In ...

2

A gamma-ray burst is rather more likely than false vacuum - we observe them on a regular (daily) basis. If one happens in our galaxy, and it's pointed at us, we had a good run. The mechanics are fairly simple - massive star collapses, huge amount of energy squirts out in 2 directions, not affected by $1/r^2$ spherical expansion. Anything in the way gets ...

2

I think there are two key misconception you seem to be having here. Before I say what it is, a disclaimer: you haven't filled in your profile, so I have no idea of your level of knowledge. Therefore, if my answer seems to be telling you how to suck eggs, please be aware that I am not meaning to be condescending. The misconceptions: 1) You seem to be ...

2

What would we see just outside of it? Pure blackness or other expanding bubbles of multiverses? Outside our particle horizon we assume that everything is more or less the same like where we are, at least if the assumption of homogenity and isotropy holds. If we live in a multiverse there might also be other laws of nature beyond or horizon, but there is ...

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