Recent content by Jorrie

  1. Jorrie

    I Wheeler's Mutability principle and multiple universes?

    Yea, I suppose it may be better to say that LCDM modeling is solid, but its original hot, dense, low entropy origin is not, due to many competing models.
  2. Jorrie

    I Wheeler's Mutability principle and multiple universes?

    I would say that LCDM is solid, but it rests upon inflation, which rests on some massive assumptions...
  3. Jorrie

    I Has the evidence for anisotropy of cosmic acceleration been discussed?

    I have searched the forum a bit but could not find a discussion of this Astrophysics letter: 'Evidence for anisotropy of cosmic acceleration', Jacques Colin et. al, 18 Oct 2019. Can someone please direct me to any forum discussion?
  4. Jorrie

    B Distance traveled by light accounting for expansion

    It could be read both ways around, so I tried to answer this part of the OP question, where he asked about the scale factors from a given time interval.
  5. Jorrie

    B Which rate of Universe expansion are we currently using?

    For large scales I think we are still using the 2018 Planck Collaboration data set, with ##H_0=67.4## +- 0.5 km/s/Mpc. One must remember that the whole dataset as a unit is consistent with the LCDM model and one cannot just change one parameter, like ##H_0##, without changing at least some of...
  6. Jorrie

    B Distance traveled by light accounting for expansion

    Hmmm, OK, but I thought that the OP's concern was that if you only have a light travel time and two scale factors, how do you find one of the mentioned proper distances. The only practical way I can think of is to make Earth the receiving galaxy and then find out what the scale factor (or...
  7. Jorrie

    B Distance traveled by light accounting for expansion

    In terms of the OP question, as I understood it, I think yes. Light does not travel the proper distance in expanding space, but simply ##c\Delta t##. By the time it arrives at the receiver, the proper distance between emitter and receiver is larger than the distance light actually traveled.
  8. Jorrie

    B Distance traveled by light accounting for expansion

    Excellent piece of writing by Brian - here is the Insights link: https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
  9. Jorrie

    B Distance traveled by light accounting for expansion

    Looking at the graph of ##a## vs ##t##, what you said is actually roughly true for a large chunk of cosmic time! It is not true for the first 3 Giga years or so due to dominant radiation energy. Also not for the last 4 Gy due to the accelerated expansion, but still close. It requires a bit of...
  10. Jorrie

    B Distance traveled by light accounting for expansion

    In a cosmos with radiation, matter and a cosmological constant (like ours), it is not straightforward to find the expansion factor change from a cosmological time interval, except approximately (if the latter is small and the expansion close to linear over time). As you expected, the general...
  11. Jorrie

    I Why does an Empty Universe have to obey Negative Curvature?

    Oops yea, I goofed the words. Thanks for correction.
  12. Jorrie

    I Why does an Empty Universe have to obey Negative Curvature?

    Nope, FLRW is a spacetime metric, because H has time in it: ##H = \frac{\dot {a}}{a}##. You must distinguish between curved space and curved spacetime. Minkowski spacetime is flat, because it does not expand: ##\kappa=0## refers to zero spatial curvature, but you must also have ##\dot {a} =...
  13. Jorrie

    I Why does an Empty Universe have to obey Negative Curvature?

    Without going into the technical stuff, as I understand it, a Milne universe is spatially flat, but has negative spacetime curvature, because the (empty) space is expanding at a constant rate (##\ddot a = 0)##.
  14. Jorrie

    I Calculate mass from time dilation

    I think the Op's interest is not velocity time dilation, but rather converting the (static) gravitational time dilation at a distance r from a large prime mass, to the actual mass of the prime. It is easily coming out of $$d\tau/dt = \sqrt{1-2M/r}$$ provided that ##d\tau/dt## is known, but how...
  15. Jorrie

    I Calculate mass from time dilation

    Peter has given you enough info in #4 above to do what you have asked - check again. But this is a very round-about way for obtaining the mass of an astronomical object. Normally one would simply observe the period and eccentricity of any object orbiting the prime mass from some distance...
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