Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

STR and the age of the universe

  1. Nov 18, 2014 #1
    Current measurements of the age of the universe are 13.798±0.037 within the Lamda-CDM concordance model. My question is how is time measured given the relativity of time? Will all observers within the universe come to the same conclusion if they assume they are at rest? Is there are a way to measure the age of the universe in terms of how much distance light has traveled since the Big Bang? But then again how would we come up with a common measuring stick to measure the distance light has traveled.?
  2. jcsd
  3. Nov 18, 2014 #2


    User Avatar
    Gold Member

    The common standard is to use "co-moving with the CMB" as the standard time, since it can be agreed on throughout the universe. The Earth is, on a cosmological scale, pretty close to co-moving w/ the CMB, but not quite.
  4. Nov 18, 2014 #3
    I don't really see why the CMB would move. How is that possible? What is going on?
  5. Nov 18, 2014 #4


    User Avatar
    Gold Member

    Exactly. That's why motion RELATIVE to the CMB is used. If you are on a planet from which (unlike Earth) you can perceived no difference in the frequency of the CMB in any direction then you are co-moving with it. Our solar system has some motion relative to the CMB so for us a tiny bit less time has passed since the big bang than it has for a co-moving planet, but we can (and DO) take that into account when stating the age of the universe.
  6. Nov 18, 2014 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    pHinds answered, here are some extras in case you or anyone else wants them
    Solar system speed relative to CMB rest frame is around 370 km/s in direction of constellation Leo (a spring constellation).

    the opposite direction, i.e. BEHIND us, is Aquarius constellation (a fall constellation, Sept. Oct Nov evenings)

    good web resource is what Ned Wright has to say. He was part of a team that made one of first accurate measurements of the CMB dipole
    (dipole = doppler hot spot in Leo, cold spot in Aquarius, )
    google "CMB dipole" and scroll down to ...ucla.edu/wright... link, which is:

    Don't get bogged in Wright's technicalities. He is a scientist and his specialty is cosmology, especially CMB. Notice he says 368 km/s and average temp 2.725 Kelvin, and
    doppler hotspot 0.00335 Kelvin hotter, coldspot 0.00335 Kelvin colder

    He gives coordinates of the dipole, but he doesn't say what constellation it is in and what time of year you can spot the direction the solar system is going.

    Notice that 0.00335/2.725 is the same fraction that 368 km/s is of the speed of light. The temperature is doppler raised by the same fraction as the speed is of the speed of light.

    they measure the sky temperature with a horn radio antenna because the heat glow is down in the infrared/microwave.


    This popular nontechnical account mentions the two constellations that mark the directions of the solar system speed in the sky
    It also points out that the solar system speed is due IN PART to the motion of our galaxy, and it is also the result of our solar system circulating WITHIN the galaxy. The galaxy is whizzing one way and the sun and planets are whizzing another way and they partly cancel and the result is this 368 km/s I've been talking about. That is what we measure, by the doppler effect.
    Last edited: Nov 18, 2014
  7. Nov 18, 2014 #6
    I'm having some difficulty understanding this. In graphic depictions of the CMB is portrayed as surrounding us on all sides because everywhere we look it is there. Now it seems logically impossible to beyond the CMB like a prisoner gets beyond his cell walls, since the CMB show the universe as it was circa 378,000 years after the BB. So I don't see how you could move with respect to an event that happened in the past.
  8. Nov 18, 2014 #7


    User Avatar
    Science Advisor
    Gold Member

    The CMB is composed of relic photons from 370,000 years after the BB, but they are still around today, which is why we can see them! They are still flying willy nilly all throughout the universe in all different directions. They bathe the universe in a radiation bath. The frame in which this radiation bath is isotropic (looks the same in all directions) is "co-moving with the CMB". The frame of the Earth moves with respect to this frame at a rate of ~400km/s.
  9. Nov 18, 2014 #8


    Staff: Mentor

    The CMB is electromagnetic radiation. It moves at the speed of light, like all electromagnetic radiation. "Motion" relative to the CMB, as other posters have explained, means that the radiation does not look isotropic; it's not the same in all directions. If it is the same in all directions, then you are "at rest" relative to it.

    If it still seems weird to you to say you're "moving" or "at rest" relative to the CMB, you can express the same physics, somewhat more verbosely, as being "moving" or "at rest" relative to an observer who sees the CMB as isotropic.

    You're not. The word "moving" here is being used in a different sense than you're used to. See above.
  10. Nov 18, 2014 #9


    User Avatar

    Staff: Mentor

    We don't. We're moving with respect to the radiation emitted by that event that happened in the past.
  11. Nov 19, 2014 #10


    User Avatar
    Science Advisor
    Gold Member

    Keep in mind confusion mainly arises due to expansion of the universe. The CMB photons we observe NOW were emitted from a distance THEN of merely 42 million light years. CMB photons emitted from our location back THEN are obviously impossible to observe NOW because they sped away about 13.8 billion years ago. Those photons emitted at a distance THEN of 42 million light years required 13.8 billion years to catch up with us due to expansion. That particular region is at a distance NOW of about 45 billion light years
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - universe Date
B The beginning of the Universe Saturday at 8:24 PM
I Is dark energy the inflow of a universal black hole? Saturday at 10:35 AM
A Zero-energy Universe Friday at 5:36 AM
B Universe versus Known Universe Mar 7, 2018
I Universe's First Stars Interaction with Dark Matter Feb 28, 2018