Is the Universe's Expansion Rate Actually Slowing Down?

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SUMMARY

The discussion centers on the expansion rate of the universe and whether it is slowing down. Key points include the observation by Adam Riess, who determined the universe is expanding at a rate of 74.2 ±3.6 kilometers/second/megaparsec. The conversation clarifies that while distant galaxies appear to be moving away faster, this does not necessarily indicate an acceleration in the expansion rate. Instead, it highlights the distinction between velocity and acceleration in cosmic terms, emphasizing that the universe underwent an inflationary period followed by a slowdown, leading to current accelerated expansion.

PREREQUISITES
  • Understanding of cosmic expansion and Hubble's Law
  • Familiarity with the concept of dark energy
  • Knowledge of astronomical measurements, specifically megaparsecs
  • Basic grasp of velocity versus acceleration in physics
NEXT STEPS
  • Research Adam Riess's findings on cosmic expansion and the implications of his measurements
  • Explore the role of dark energy in the universe's expansion
  • Study the differences between velocity and acceleration in cosmology
  • Investigate the historical context of the universe's inflationary period
USEFUL FOR

Astronomers, astrophysicists, and students of cosmology who are interested in understanding the dynamics of the universe's expansion and the implications of recent measurements on cosmic theories.

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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