When Did the Universe Start Accelerating?

June_cosmo
Messages
24
Reaction score
0

Homework Statement


Assume the cosmological model with H0=72,Omega_M=1-Omega_lamda=0.3,(so dark energy with w = − 1) and a flat universe.)
a) Find the redshift z at which the universe starts accelerating (that is, when it transitions from decelerating to accelerating).
b) How long ago did this happen?

Homework Equations

The Attempt at a Solution


I don't quite understand the accelerating here. Does it mean that the expanding rate begins to accelerate? How should I start this problem?
 
Physics news on Phys.org
Accelerating expansion means that the second derivative of the scale factor ##a(t)## is positive.
 
Orodruin said:
Accelerating expansion means that the second derivative of the scale factor ##a(t)## is positive.
Orodruin said:
Accelerating expansion means that the second derivative of the scale factor ##a(t)## is positive.
I see. Should I use the deceleration formula?
 

Thread 'Initial conditions for orbits around a wormhole'

Hi, I had an exam and I completely messed up a problem. Especially one part which was necessary for the rest of the problem. Basically, I have a wormhole metric: $$(ds)^2 = -(dt)^2 + (dr)^2 + (r^2 + b^2)( (d\theta)^2 + sin^2 \theta (d\phi)^2 )$$ Where ##b=1## with an orbit only in the equatorial plane. We also know from the question that the orbit must satisfy this relationship: $$\varepsilon = \frac{1}{2} (\frac{dr}{d\tau})^2 + V_{eff}(r)$$ Ultimately, I was tasked to find the initial...
View full post »

Thread 'Help to convert units of a simple formula'

The value of H equals ## 10^{3}## in natural units, According to : https://en.wikipedia.org/wiki/Natural_units, ## t \sim 10^{-21} sec = 10^{21} Hz ##, and since ## \text{GeV} \sim 10^{24} \text{Hz } ##, ## GeV \sim 10^{24} \times 10^{-21} = 10^3 ## in natural units. So is this conversion correct? Also in the above formula, can I convert H to that natural units , since it’s a constant, while keeping k in Hz ?
View full post »

Similar threads

I Age of the Universe and the Cosmological Constant
Replies
17
Views
2K
Link redshift with luminosity distance?
Replies
3
Views
2K
At what redshift did the Universe beging accelerating?
Replies
1
Views
3K
I Receding movement of a stopped object in an accelerating universe?
Replies
23
Views
2K
I How do you get to the conclusion that there is accelerating expansion?
Replies
23
Views
2K
I Energy of Light in Accelerating Universe: Comparing Cases
Replies
24
Views
2K
B Cosmological constant for the Universe's expansion
Replies
5
Views
2K
How Does the Cosmological Constant Change with the Expansion of the Universe?
Replies
2
Views
2K
A Evolution of radio sources mimics a non-expanding universe
Replies
1
Views
2K
I Dark energy and the kinematic Sunyaev-Zeldovich effect?
Replies
22
Views
2K

Hot Threads

Recent Insights

Back
Top