# Tag Info

12

There certainly are quasars. Obviously, we can't see any as they are today because they aren't nearby. Quasars are a type of Active Galactic Nucleus(AGN), which means we won't find any within our local few million lightyears. However, as the Wikipedia page on Quasars will tell you, any time a supermassive black hole of a galactic nucleus gets a massive ...

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These types of theories that physicists such as Krauss espouse of a "Universe Coming From Nothing" are quite flawed, as by no means are they talking about nothing! Further, the concepts of particles, mass, and energy are not even well-defined when talking about the universe in general. I wrote a paper on this (excuse the shameless self-promotion), it can be ...

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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 ...

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Enzo is fundamentally a grid-based finite-volume hydrodynamics code. That is, the domain is divided into cells, each is assigned various fluid properties (density, velocity, etc.), and at each timestep fluxes of those quantities across the interfaces between cells are used to update the quantities in the cells. It has a choice of particular methods for ...

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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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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 ...

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I think you may be confusing two "versions" of the cosmological constant. First there is $\Lambda$ (or sometimes $\lambda$), which expresses the absolute energy density of "dark energy". Depending on convention, this constant can have a variety of dimensions. You seem to have the dimensionless (or Planck unit) version in mind, which is: ...

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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 ...

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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 ...

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} + ... 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 ... 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 ... 2 there are many experiments that put different contraints on the neutrino masses. Here is a good collection from the particle data group. 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 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 ... 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 ... 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 ... 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 ... 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 ... 1 It can be shown that \omega_{ab} = 0 \Leftrightarrow \omega^a \equiv \epsilon^{abcd}u_b \nabla_c u_d = 0. The latter quantity is known as the twist (or vorticity). In a local inertial frame it is easy to see that \vec{\omega} \sim \vec{\nabla}\times \vec{v} where \vec{v} is the 3-velocity field of the flow. This lends to the following interpretation ... 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 ...

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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.)

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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: ...

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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.

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