Accelerating universe explained by constant expansion?

Click For Summary
SUMMARY

The discussion clarifies the distinction between constant expansion and accelerating expansion in the universe. It emphasizes that while the universe can expand at a constant rate, particles within it will perceive each other as accelerating away due to the nature of the expansion. The key takeaway is that accelerating expansion is characterized by a positive second derivative of the scale factor, while constant expansion rate leads to an exponential growth of the scale factor, indicating an accelerated expansion. In contrast, a non-accelerating (coasting) universe features a linear growth of the scale factor with a decreasing Hubble parameter over time.

PREREQUISITES
  • Understanding of the Hubble parameter and its implications in cosmology
  • Familiarity with the concepts of scale factor and its derivatives in the context of universal expansion
  • Knowledge of the differences between constant and accelerating expansion in cosmological models
  • Basic grasp of the implications of particle dynamics in an expanding universe
NEXT STEPS
  • Research the mathematical formulation of the scale factor in cosmology
  • Study the implications of the Friedmann equations on universal expansion
  • Explore the concept of dark energy and its role in accelerating expansion
  • Learn about observational evidence supporting the accelerating expansion of the universe
USEFUL FOR

Astronomers, physicists, cosmologists, and students interested in the dynamics of the universe and the principles of cosmic expansion.

RussB
Messages
7
Reaction score
6
Why is the accelerating expansion of the universe not explained simply by the perspective of acceleration caused by a constant (non accelerating) universal expansion? Let's take two particles (X,Y) in a 1d expanding universe where it expands universally by 1 unit per per unit time. O represents empty space.

T=0: XY
T=1: XOY
T=2: XOOOY
T=3: XOOOOOOOOOY

Even though the universe is expanding in a completely constant way, any given particle would perceive every other particle (not bound by some sufficiently strong force) to be accelerating away.
 
Astronomy news on Phys.org
What you are describing is constant expansion rate, not constant expansion. Accelerating expansion means that the second derivative of the scale factor is positive. Constant expansion rate (Hubble parameter) means that the scale factor grows exponentially and hence corresponds to an accelerated expansion - as you have deduced. In a non accelerating expansion (coasting universe), the scale factor grows linearly and the Hubble parameter is decreasing with time.
 

Similar threads

Undergrad How Does the Balloon Analogy Explain the Expansion of the Universe?
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad How fast were distant galaxies moving?
  • · Replies 7 ·
Replies
7
Views
2K
High School The accelerating expansion of the universe
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad On Hubble's constant, the expansion of the Universe and vacuum energy
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Question about the cosmological constant
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Age of the Universe and the Cosmological Constant
  • · Replies 18 ·
Replies
18
Views
3K
Undergrad What happens to spacetime during the expansion of the Universe?
  • · Replies 21 ·
Replies
21
Views
3K
High School Distance of Stars / Size of the Universe calculation
  • · Replies 6 ·
Replies
6
Views
3K
Undergrad Who really found out the Universe's acceleration?
  • · Replies 7 ·
Replies
7
Views
2K
Graduate How Do We Know the Universe's Expansion Is Accelerating?
  • · Replies 3 ·
Replies
3
Views
2K