17 Matching Annotations
  1. Nov 2019
    1. After all, aside from the Planck data, the lambda-CDM model, which is the standard model of the universe, seems to work really well. Using just six parameters, it seems to fit our observations of the universe, albeit a flat universe, nearly perfectly.

      I'm not sure what the difference is between a flat or an open universe

  2. Dec 2018
    1. Neutrino Properties and the Cosmological Tensions in the ΛCDMMode

      By Stefano Gariazzo

  3. Nov 2018
    1. e-fold of cosmic expansion

      Exponential expansion means that all cosmic distances will increase by a factor e (=2.718) every 17.3 billion years (or 17.3 Gy for short). This can be called the natural ‘e-fold time’ of our universe, since an increase by a factor e is called an e-folding. (see: https://hyp.is/7Az4duONEeilMUPe6XwMOA/www.physicsforums.com/insights/approximate-lcdm-expansion-simplified-math/)

    1. We review three distance measurement techniques beyond the local universe: (1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and (3) HI intensity mapping. We describe the principles and theory behind each method, the ingredients needed for measuring such distances, the current observational results, and future prospects. Time delays from strongly lensed quasars currently provide constraints on H0H_0 with < 4% uncertainty, and with 1% within reach from ongoing surveys and efforts. Recent exciting discoveries of strongly lensed supernovae hold great promise for time-delay cosmography. BAO features have been detected in redshift surveys up to z <~ 0.8 with galaxies and z ~ 2 with Ly-αα forest, providing precise distance measurements and H0H_0 with < 2% uncertainty in flat ΛΛCDM. Future BAO surveys will probe the distance scale with percent-level precision. HI intensity mapping has great potential to map BAO distances at z ~ 0.8 and beyond with precisions of a few percent. The next years ahead will be exciting as various cosmological probes reach 1% uncertainty in determining H0H_0, to assess the current tension in H0H_0 measurements that could indicate new physics.

      Review article accepted for publication in Space Science Reviews (Springer), 45 pages, 10 figures. Chapter of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination

    1. The ‘radiation negligible’ approximation is pretty good from about 100 million years after the start of expansion, when the radiation energy density dropped to less than 1% of the matter energy density.

      Omega_r vs Omega_m

    2. The traditional unit of the Hubble constant as used by Edwin Hubble is kilometers per second per Megaparsec. From an educational p.o.v. it was an unfortunate choice, because it seems to imply a recession speed, while it is really a fractional rate of increase of distance. It is a distance divided by a distance, all divided by time. So its natural unit is 1/time, for which we can choose any convenient timescale

      The Hubble constant is not a recession speed, it is really a fractional rate of increase of distance.

    3. Exponential expansion means that all cosmic distances will increase by a factor e (=2.718) every 17.3 billion years (or 17.3 Gy for short). This can be called the natural ‘e-fold time’ of our universe, since an increase by a factor e is called an e-folding.

      Definition of: e-folding

    1. the ‘particle horizon’

      the proper distance that a massless particle, emitted at cosmological time zero, could have bridged until the time it is observed, as judged by the observer

      The only difference between the equation for the particle horizon and proper distance today is in the limits of the integral – for D_par we have to integrate from today (S=1) all the way to S approaching infinity (as time is extrapolated back to time zero, redshift would extrapolate to infinity)

    2. the cosmological communications horizon (also called the cosmic event horizon), which is how distant an observer can be and still receive a signal that we transmit today

      Contrast this to D_par, which is about information emitted in our past, reaching us today.

    3. Hubble radius. It is the distance at which the recession rate of an object equals the speed of light. In our natural units, it is simply the inverse of the Hubble constant H at the time.
    1. cosmological recession rates are quite like the growth rate of money in an investment, which we normally express as a percentage return over a period of time. But, we can also express it as so many dollars earned per time period, which is essentially the present ‘speed of growth’ of the investment. Cosmic recession rates are similar to this sort of speed.The proper distance of a galaxy is equivalent to the investment amount, the Hubble constant is equivalent to the interest rate and the increase in proper distance is equivalent to the dollars earned. Divide the increase in proper distance (say km) by a time period (say seconds) and you have a recession rate in km/s. The larger the proper distance, the larger the recession rate, without any upper limit.

      This page has text and formulas for past, present, future, and generic recession rates.

    2. Suppose there are a zillion identical, straight blue bars supporting a zillion red cubes. Also suppose that due to some mechanism, each blue bar is continuously lengthening at a rate of one ‘red cube side length’ per year, while all the cubes remain unaffected, size-wise.
  4. Oct 2018
  5. Jun 2016
  6. arxiv.org arxiv.org
    1. Here, we prove thatarbitrarilyinhomo-geneous and anisotropic cosmologies with “flat” (includingtoroidal) and “open” (including compacthyperbolic) spatial topology that are initially expandingmust continue to expand forever at least insome region at a rate bounded from below by a positive number,despite the presence of arbitrarilylarge density fluctuations and/or the formation of black holes

      excellent talk by Matt Kleban on this in Utrecht, conclusion "There is no initial condition for inflation" in opposition to Steinhardt claims

  7. Feb 2016
  8. arxiv.org arxiv.org
    1. We describethe minimal modification required for self-acceleration and show that its maximum likelihood yieldsa 2.4σpoorer fit to cosmological observations compared to a cosmological constant, which, althoughmarginally still possible, questions the concept of cosmicacceleration from a genuine scalar-tensormodification of gravity.

      The observed GRB might not be connected, but still very interesting to ask what the implications of proving c_T=c_s to high precision. Future direct detection of GW with confirmed optical follow up will surely come soon.

  9. Dec 2015
  10. Nov 2015
    1. Spherically Symmetric N-body Simulations with General Relativistic Dynamics

      Everyone can see this though...