# Tag Info

18

Yes, definitely. The Hubble constant describes the expansion rate of the Universe, and the expansion may, in turn, may be decelerated by "regular" matter/energy, and accelerated by dark energy. It's more or less the norm to use the term Hubble constant $H_0$ for the value today, and Hubble parameter $H(t)$ or $H(a)$ for the value at a time $t$ or, ...

16

If you measure the gravitational waveform from an inspiralling binary, you can at any point measure the amplitude, instantaneous frequency and the rate of change of frequency. The last two give you the "chirp mass", which is related to the product and sum of the binary component masses. The amplitude of the gravitational wave then depends on the chirp mass ...

15

To make a long story short, the measurements from Planck and the Hubble Space Telescope disagree, and the reason behind this isn't known. First, let's look at the values with the uncertainties. We then have three different results that are, perhaps, not as inconsistent as they originally seemed: $70.0^{+12.0}_{-8.0}\text{ km s}^{-1}\text{ Mpc}^{-1}$ from ...

12

Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 results from the Planck observations of the cosmic microwave background is probably the one that most people will accept, but even in that paper you will find ...

12

We can currently only detect gravitational radiation when it is extremely intense: in the last fraction of a second. For example the first gravitational wave detection lasted less 0.15 seconds. The black holes are releasing gravitational radiation with every orbit, but that radiation is too weak for us to detect. It takes a colossal amount of energy being ...

10

The Sloan Digital Sky Survey Data Release 15 contains over 4 million spectra of both galactic and extra-galactic origin from the multi-fiber spectrographs. Of these spectra, 0.7 million came from the original spectrographs during the SDSS-I/II Legacy Survey and the remainder from the upgraded spectrographs as part of the BOSS survey during SDSS-III (see SDSS ...

9

So where are these measurements of galaxies moving faster than light? They're redshift measurements. Check out the Wikipedia redshift article. It's good stuff. "we can actually observe galaxies that are moving away from us at >c" It's true. You might think it cannot be, but it can. Um, I think I missed the groundbreaking headline that said ...

8

The Hubble parameter is defined as the rate of change of the distance between two points in the universe, divided by the distance between those two points. The Hubble parameter is getting smaller because the denominator is getting bigger more quickly than the numerator. In the future, the cosmological constant, $\Lambda$ could result in an exponential ...

8

The Hubble law gives the velocity of a distant galaxy right now. A galaxy at a distance $d$ recedes at a velocity $v = H_0\,d$ right now$^\dagger$. However, the relation between $d$ and the redshift — which is the quantity that we observe — is a non-trivial function of the expansion history of the Universe, obtained by integrating the (inverse) scale factor ...

8

At a distance of $d = 87\,\mathrm{Mpc}$, with a Hubble constant of roughly $H_0 = 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ cosmological expansion should make the host galaxy UGC 11723 recede at $v=H_0 \,d\simeq6100 \,\mathrm{km}\,\mathrm{s}^{-1}$. However, galaxies also move through space, at typical velocities from several $100\,\mathrm{km}\,\... 7 Hubble's law is a bit more subtle than you suppose and an expansion, whether accelerating or decelerating does not invalidate it. The distance and speed that should be used are their values now. These are known as the proper distance and velocity respectively. In that form Hubble's law works just fine, providing that the cosmological principle - that the ... 7 Just a supplement to @JamesK's excellent answer. The image below (from Caltech/MIT by way of New Sciencist) shows what was detected for one collision. On the left (at the start) the blackholes orbit one another about every 0.03 seconds, but the waveform is too faint to detect. At about 0.3 seconds on the Time axis the waves start being detectable and the ... 7 The duration of a gravitational wave detection is not particularly important in detecting electromagnetic counterparts, although the fact that they are not recurrent or repeating sources is. Binary systems continually emit gravitational waves, up until the time that they merge, predominantly at twice the orbital frequency. At the same time, the power emitted ... 7 Think about it as if you were baking bread (or cake, whatever you prefer). When you bake a bread with raisins, it rises in all directions (due to the yeast). Every raisin in a rising loaf of raisin bread will see every other raisin expanding away from it. Now suppose the raisins as galaxies and the yeast as the Hubble constant. An illustration from ... 6 What the Hubble constant really depends on is how old was the universe at the time, but if you have a dynamical model of the universe, you can map that into z and come up with a function H(z). So in that sense, the answer is "yes," but be careful-- we also think of z as a measure of how far away the objects are, and H does not depend on location it depends ... 5 A finite universe is said to have a "closed geometry", or to be "positively curved", meaning that, in principle, you may travel in a straight line and eventually return back to your starting point. In the 2D analogy, the surface of Earth is positively curved, and if you travel 40,000 km straight, you're back where you started. A finite universe that does ... 5 A type 1a supernova forms when a white dwarf grows through accretion to a certain size, at which it becomes unstable. This means that the precursor object is always a white dwarf of mass 1.39 solar masses. As the precursor object is always of the same type and the same size, the supernova is thought to be the same brightness. On the other hand, type II ... 5 Am I naive to think that it is necessary to build up a nice, complete light curve with dense points in time in order to use the photometry for precision distance calculations? No, the more I read about it, the more difficult it seems to be. In principle, if you sample regularly and often, you should eventually get a light curve, unless the period is the ... 4 Without going into the technicalities of spacetime diagrams and ants, I think the quickest way to wrap your head around this is to look at it from the distant galaxy's perspective. For instance, let's take GN-z11, which actually receded from us at$v\simeq4c$when it emitted the light we see today: A photon left GN-z11 at$v=c$. Space expands, so although ... 4 No, only the Hubble law was recommended to have its name changed (I'm a member of the IAU, so hopefully I'd have known if there were more votings). However, several astronomers (including myself) found the voting a bit… weird; while acknowledging the work of George Lemaître is admirable, many more people than him and Edwin Hubble contributed to the ... 4 Hubble tension refers to the incompatibility between different measurements of the value of the Hubble constant. These measurements are incompatible up to more than$5 \sigma$. This incompatibility arises between what we measure "nearby" and what we measure further away, and indicates that there might be some physics we don't understand yet. Now there has ... 4 Here are some galaxies (in the first line), and the same galaxies a bit later (in the second line) If you are in the red galaxy (the one in the middle), galaxies that used to be one space away are now two spaces away, so they are moving away with a speed of one space per unit time. Galaxies that used to be two spaces away are now four spaces away, so they ... 3 Here is some remarks on the issue, straight from Ryden: If galaxies are currently moving away from each other, then it implies they were closer together in the past. Consider a pair of galaxies currently separated by a distance$r$, with a velocity$v = H_0r$relative to each other. If there are no forces acting to accelerate or decelerate their ... 3 While the Hubble constant describes the current expansion rate of the Universe, it should also be seen as a parameter (among others) of a given cosmological model (e.g., Lambda-CDM). All methods to measure the Hubble constant are more or less indirect in some way, and they rely on very different assumptions. For example, the emission of the Cosmic Microwave ... 3 Casertano et al. used the period-luminosity (P-L) relation of Cepheid variables as a sanity check on Gaia DR1 parallaxes. They chose Cepheid variables within the Milky Way having parallaxes in TGAS (Tycho-Gaia astrometric solution, reusing Hipparcos data for a head start). The period and apparent magnitude come from ground-based photometry (van Leeuwen et al.... 3 The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch. I hope this helps. https://arxiv.org/pdf/1907.05922.pdf 3 The simplest answer is that Milne's explanation requires a non-uniform cosmos, with a formerly crowded "starting area" that expands into empty space. Observations indicate that the cosmos is nearly uniform at very large scales. With Milne's model of a non-expanding space and uniform cosmos, galaxies that started near to us with high velocities that ended ... 3 This answer to the question “Is the Hubble constant dependent on redshift?” gives the formula (a form of the Friedmann equation) for the Hubble parameter$H(z)$as a function of redshift$z$: $$H(z)^2 = H_0^2 \left[ (1+z)^4 \Omega_r + (1+z)^3 \Omega_M + (1+z)^2 \Omega_k + \Omega_\Lambda \right]$$ where the$\Omega$terms are the fractional densities in ... 3 No. Time dilation is expected and observed as a result of universal expansion (e.g. in the light curves of type Ia supernovae and the duration of gamma ray bursts, Blondin et al. 2008; Zhang et al. 2013). There is no way of distinguishing between a Doppler shift and cosmological redshift for an individual source. 2 All of these answers start with the formula and say therefore$H_0\$ decreases, but I think I have a more intuitive explanation: If you imagine the universe as an open straight rubber band, and the expansion of the universe stretching markings (galaxies) on the band, and say one end is being held stationary (this is us), then the rate at which each galaxy ...

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