# Tag Info

18

Interstellar space is an excellent vacuum, but it's not a perfect vacuum. For example Earth is constantly bombarded with protons from the solar wind, which stream outward uninterrupted until the heliopause when matter from other stars becomes more dominant. If there were, say, an antimatter star nearby, the place where its stellar wind of antiparticles met ...

8

There are a few big differences: Tidal effects change the rate at which the orbit decays The neutron stars touch before the black holes would have merged Ejected matter can contribute to the gravitational-wave signal The merged neutron star can have "mountains" that keep radiating Probably the most important effect: the matter in NSs can emit photons and ...

7

The Big Bang Theory is a much more general and less specific description of our theory about the origin of the Universe than the $\Lambda{\rm CDM}$ model (by the way, I don't think that the hyphen is written in that acronym). The Big Bang Theory says that the Universe was expanding and the distances between two places where galaxies sit today used to be ...

6

A typical velocity dispersion in a globular cluster is 10 km/s. For a typical 1 solar mass subgiant in an old globular, then equating the kinetic energy to $3kT/2$, we get $T = 5\times 10^{60}$ K. Doesn't seem that helpful really... The concept of temperature is only ever applied in a relative sense - i.e. some component is hotter than another. Can't say ...

6

There are several things going on and you seem to have a good grasp of the factors involved. White dwarfs are produced from progenitors with main sequence masses between around 8 solar masses (any more massive and it leads to a supernova and a neutron star) at the top end and about 1 solar mass at the bottom end. This lower limit is nothing fundamental, it ...

4

The galactic magnetic field is fairly irregular on distance scales that are small compared to the size of the galaxy (although there does appear to be structure to the magnetic field associated with the spiral arms). In a uniform magnetic field, a charged particle would follow a nice spiral trajectory. In an uneven and varying magnetic field, charged ...

3

Space is, as you say, good for removing a lot of the background noise that spoils LIGO's data — like seismic noise, disturbances from traffic and logging activity, people shooting at the beam tubes, etc. But another important reason to go into space is so that you can basically make a much larger version of LIGO. LIGO's arms are 4km long; eLISA's arms will ...

3

This is an active (hot?) topic of research, in fact I attended a workshop on the subject just last week. In brief, no one has found a dark matter (DM) halo yet that does not host a galaxy, though we would very very much like to! The first reason it's so difficult to find a DM halo that does not have a galaxy is that a common working definition of a galaxy ...

3

A massive object (such as a galaxy) along the line of sight to a distant bright source (such as a quasar) bends the light along its path. If the "lensing" object is massive enough and the geometry is right, the background object can be seen as multiple sources. For instance, here is a galaxy (central point) and four images of a single quasar: For a ...

2

There are binary stars (orbiting around their centre of mass) and there are stars orbiting around neutron stars or black holes (or rather, again, around the centre of mass of the system). I don't think many stars would orbit a black hole, except... There is the black hole at the centre of most galaxies, including our own. Lots of stars orbit around that - ...

2

The correct formula is actually $$M = \frac{4\pi^2 a^3}{GP^2}$$ and is a form of Kepler's third law. $M$ in this formula is the central mass which must be much larger than the mass of the orbiting body in order to apply the law. In reality the formula that should be used is $$M_1 + M_2 = \frac{4\pi^2 a^3}{GP^2},$$ where $M_1+ M_2$ is the sum of the masses ...

2

The vast majority of the star like objects we see in the sky are stars in our own galaxy. Assuming the accelerated expansion is due to a cosmological constant, and assuming the value of the cosmological constant does change (it's currently of order $10^{-52}\,\text{m}^2$) the expansion will never be strong enough to disrupt the Milky Way. So our night sky is ...

2

Great question. Black holes are some of the brightest objects in the universe. While we think they require the Blandford-Znajek (BZ) mechanisms to produce things like Relativistic Jets, the bulk of the light (emission) they produce is just the efficient thermalization of gravitational energy when material falls into (`accretes' onto) them. The simplest way ...

2

Temperature is not useful concept for describing clusters of stars or other gravitational systems, because such systems are not in the realm described by thermodynamics. There is no way to set up thermodynamic equilibrium - globular clusters partly evaporate and core implodes. Also the velocity distribution can't be Maxwell-Boltzmannian, because very fast ...

2

The important point here is that there is no thermodynamic limit for gravitating systems, and thus there is no well-defined temperature. This is, perhaps, not a completely intuitive result, but it comes from work on the stability of matter. This is not as glamorous as it sounds, but revolves around the need to show that the energy of matter is an extensive ...

1

Mike's answer is good but there Is more. You can actually see it in the papaer he referred you to, it is an excellent paper. It not only describes the possible sources, but it points to the new physics that it might see. eLisa, also called NGO, will be sensitive to gravitational waves from the early universe (and after), down to 10 to the minus 18 sec after ...

1

The fact that the gyroradius is small compared to the Galaxy size leads to a multitude of collisions between the CR and the galactic magnetic field (compare ~pc CR vs ~kpc CR gyroradii). Each collision helps diffuse the particle, disassociating it from its original direction (i.e., makes isotropic).

1

Firstly, you must know that there are many models for inflation which give different results to your a) and b) questions, and we still don't know which is the right one. I'll try to answer regarding the most accepted and simple models. a) During the period of inflation the distance between two separated points in the Universe increased at least ...

1

Note You should clarify your statement from "...a charged particle cannot gain energy from a magnetic field..." to "...a charged particle cannot gain energy from a static magnetic field..." There is nothing wrong with energy transfer from time-varying magnetic fields. Background If the spatial gradient in the magnetic field is slow enough such that the ...

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