Is the Universe's Expansion Rate Actually Slowing Down?

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Discussion Overview

The discussion centers on the rate of the universe's expansion and whether it is actually slowing down or accelerating. Participants explore concepts related to cosmic expansion, dark energy, and observational measurements, engaging with both theoretical and observational aspects of cosmology.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that observing distant objects implies they were moving apart faster in the past, potentially indicating a slowing expansion rate.
  • Another participant counters that the relationship between recession velocity and distance does not imply acceleration without breaking the simple proportionality, clarifying that increasing velocity does not necessarily mean increasing acceleration.
  • A different participant mentions a historical inflationary period followed by a current phase of accelerated expansion, referencing measurements by Adam Riess regarding the universe's expansion rate.
  • There is a claim about the expansion rate being measured in terms of speed, which is challenged by another participant who states that expansion cannot be measured in speed units as it depends on distance.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the universe's expansion, with some asserting that it is accelerating while others question the interpretation of observational data. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

There are limitations in the assumptions made regarding the relationship between distance and recession velocity, as well as the interpretation of acceleration versus velocity in the context of cosmic expansion.

saywhat
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Most articles sate that basically the further away an object is, the faster it is moving away from us therefore the universe is expanding faster which leads to dark energy to explain the increasing accelaration
But as we see distant objects as they were in the past due to the finite speed of light isn't this also equivalent to saying :
"when we look further back in time, things were accelerating apart faster then they are now" which implies the rate of expansion is slowing?
thanks.
 
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saywhat said:
Most articles sate that basically the further away an object is, the faster it is moving away from us therefore the universe is expanding faster which leads to dark energy to explain the increasing accelaration

You don't quite have this right. The proportionality of recession velocity to distance just shows that the universe is expanding. To show that the expansion is accelerating requires breaking the simple proportionality. You're using "increasing acceleration" to mean "increasing velocity." Velocity increases even if the acceleration is constant.
 
the universe had a inflationary period when it expanded very fast.then it slowed down.presently the universe is expanding with increased acceleration.
the expansion rate is about 2 to 20 times speed of light.(not sure)

Adam Riess and his team observed quite a few galaxies in this way, and figured just how fast the Universe is growing to unprecedented accuracy. His result: 74.2 ±3.6 kilometers/second/megaparsec. That means for every megaparsec (about 3 million light years) you go out, the Universe is expanding 74.2 km/sec faster. So a galaxy 10 Mpc away would be moving away from us at 742 km/sec. Adam’s measurement jibes well with other measurements, so there is reason to be confident in his results.

http://blogs.discovermagazine.com/badastronomy/2009/05/07/the-universe-is-expanding-at-742-kmsecmpc/
 
Last edited:
vikramblue said:
the expansion rate is about 2 to 20 times speed of light.(not sure)

The expansion rate can't be measured in units of speed. The speed of one galaxy relative to another depends on distance.
 

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