How Is the Age of the Universe Measured Given Relativity and Expansion?

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Discussion Overview

The discussion centers on how the age of the universe is measured in the context of relativity and cosmic expansion. Participants explore the implications of time measurement, the Cosmic Microwave Background (CMB), and the challenges of defining a common reference frame for measuring distances in an expanding universe.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that current measurements of the universe's age are based on the Lambda-CDM model, questioning how time can be consistently measured given the relativity of time.
  • One participant suggests using "co-moving with the CMB" as a standard time reference, indicating that while Earth is close to this frame, it is not perfectly aligned.
  • Another participant expresses confusion about the motion of the CMB, questioning how it can be perceived as moving and what that means for measurements of time and distance.
  • Some participants clarify that motion relative to the CMB is used to account for differences in time passage, noting that the solar system has a specific velocity relative to the CMB.
  • One participant discusses the implications of the CMB being a remnant from the early universe, emphasizing that it represents a snapshot of the universe at a specific time and raises questions about moving with respect to it.
  • Another participant explains that the CMB consists of relic photons that are still present today, which leads to the concept of being co-moving with the CMB frame.
  • Some participants highlight that confusion arises from the universe's expansion, noting that CMB photons emitted from a certain distance in the past are now observed from a much greater distance due to this expansion.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and agreement regarding the implications of the CMB and the measurement of the universe's age. Multiple competing views remain, particularly concerning the interpretation of motion relative to the CMB and the nature of time measurement in an expanding universe.

Contextual Notes

Participants mention limitations in understanding due to the complexities of cosmic expansion and the nature of the CMB, as well as the challenges in establishing a common measuring stick for distances in the universe.

gamow99
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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.?
 
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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.
 
I don't really see why the CMB would move. How is that possible? What is going on?
 
gamow99 said:
I don't really see why the CMB would move. How is that possible? What is going on?
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.
 
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:
http://www.astro.ucla.edu/~wright/CMB-DT.html

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.

http://Earth'sky.org/constellations/aquarius-heres-your-constellation
http://Earth'sky.org/constellations/leo-heres-your-constellation
http://stardate.org/nightsky/constellations/leo

This popular nontechnical account mentions the two constellations that mark the directions of the solar system speed in the sky
http://astronomy.swin.edu.au/cosmos/C/Cosmic+Microwave+Background+Dipole
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.
 
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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.
 
gamow99 said:
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.

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.
 
gamow99 said:
I don't really see why the CMB would move.

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.

gamow99 said:
I don't see how you could move with respect to an event that happened in the past.

You're not. The word "moving" here is being used in a different sense than you're used to. See above.
 
gamow99 said:
I don't see how you could move with respect to an event that happened in the past.

We don't. We're moving with respect to the radiation emitted by that event that happened in the past.
 
  • #10
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
 

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