Hubble lookback time based on energy density.

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Homework Help Overview

The discussion revolves around calculating the Hubble lookback time in a hypothetical universe characterized by a specific energy density of the vacuum, set at 10^133 eV/m^3, with no curvature or other matter present. Participants are exploring the implications of this scenario within the context of cosmology.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster expresses uncertainty about the redshift (z) to assume and whether to convert energy density to kg/m^3. Other participants seek clarification on the definition of Hubble lookback time and its distinction from Hubble time, with some suggesting that the lookback time may not be meaningful without specifying a telescope or object.

Discussion Status

Participants are actively questioning the definitions and assumptions related to lookback time and Hubble time. Some have provided insights into the relationship between energy density and the Hubble parameter, with references to the Friedmann equation. However, there is no explicit consensus on the approach to take, and the discussion remains exploratory.

Contextual Notes

There is a noted lack of clarity regarding the assumptions necessary for the calculations, particularly concerning the definitions of lookback time and Hubble time. Participants are also considering the implications of a flat universe with zero matter density.

toughcanesrock
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Homework Statement


If the energy density of the vacuum were the value 10^133 eV/m^3 , what would the value of the Hubble lookback time be for such a universe with no curvature and no other matter?

Homework Equations


Possible equation...

...Lookback time = ln(1+z)/H

The Attempt at a Solution


I am not sure what to assume for z? And should I convert the energy density to kg/m^3. I am very confused, any direction for this question would be IMMENSELY appreciated.
 
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Tell us what you mean by Hubble lookback time.

Do you mean how far back in time a specific telescope e.g. the HST can see?
 
There is also a well-defined conventional quantity known as the Hubble time
it is not a "lookback" time and it is not associated with any particular telescope. It is just 1/H where H is the usual Hubble expansion rate "constant" (which is actually not constant over time)
The current value of the Hubble time is about 14.4 billion years.
Not to be confused with any "lookback" time.
 
Last edited:
R
marcus said:
There is also a well-defined conventional quantity known as the Hubble time
it is not a "lookback" time and it is not associated with any particular telescope. It is just 1/H where H is the usual Hubble expansion rate "constant" (which is actually not constant over time)
The current value of the Hubble time is about 14.4 billion years.
Not to be confused with any "lookback" time.

Right. So what my professor wants to know is what is the lookback time for a flat universe, with no matter, and a density of 10^133 eV/m . Would you know how to go about starting this? OmegaM would be 0 and OmegaK would be 0 right?
 
I'm not the right person to help you. In my universe "lookback" time doesn't mean anything unless you specify a telescope or an object you are looking at. If your professor were asking about the Hubble time then I would know how to proceed.

I would just multiply the energydensity by 8piG/3c^2 and that would give me H^2 and then the Hubble time is just 1/H and we're done.
It would be a simple application of the Friedmann equation and the Hubble time is constant in that situation.
 
marcus said:
I'm not the right person to help you. In my universe "lookback" time doesn't mean anything unless you specify a telescope or an object you are looking at. If your professor were asking about the Hubble time then I would know how to proceed.

I would just multiply the energydensity by 8piG/3c^2 and that would give me H^2 and then the Hubble time is just 1/H and we're done.
It would be a simple application of the Friedmann equation and the Hubble time is constant in that situation.
I think I will just do that, as that is what some of my peers are doing. Thanks so much for the assistance.
 

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