Is Hubble Constant Really Constant?

In summary, the Hubble constant, which relates recessional velocity to proper distance, is not a constant but changes with time. However, the Hubble parameter is a constant throughout space, with its value now being referred to as H0. This concept may be confusing due to the theory of relativity, but it refers to the current location in space-time.
  • #1
jayaramas
30
0
i think Hubble constant is not a constant because of relativistic speeds of galaxies. is it?
 
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  • #2
jayaramas said:
i think Hubble constant is not a constant because of relativistic speeds of galaxies. is it?

You're right that the thing we call the Hubble parameter, H, which relates recessional velocity to proper distance, is not constant. It changes with time, and so it can be expressed as a function H(t). However, your reason is wrong: it has nothing to do with "relativistic speed" of galaxies. It changes with time in a manner determined by the Friedmann equations, which describe the large-scale dynamics of the universe in terms of its matter and energy content.

The thing we we call the Hubble constant, H0, is the value of the Hubble parameter NOW. This is, in fact, a constant.

Note that although H changes with time, it is constant throughout space (i.e. it does not vary with location). Therefore, H is the same everywhere at any given instant.
 
  • #3
this is not the correct answer!. what is NOW ? this indicates lack of understanding of theory of reletivity
 
  • #4
Asking a question you already 'know' the answer to is commonly referred to as trolling. Of course, that is not an issue in this case. I suggest a refresher course on the concept of 'NOW'.
 
  • #5
jayaramas said:
this is not the correct answer!. what is NOW ?

The space time coordinate of the current location.

This indicates lack of understanding of theory of reletivity

Be very careful on this forum with asserting that people don't understand things.
 
  • #6
sorry. what i mean is that the distant galaxy is not moving at calculated speed NOW but is was moving like that long back!
 

FAQ: Is Hubble Constant Really Constant?

1. What is the Hubble Constant?

The Hubble Constant is a measure of the rate at which the universe is expanding. It is denoted by the symbol H0 and has units of kilometers per second per megaparsec.

2. Why is the Hubble Constant called a constant?

The Hubble Constant is called a constant because it was originally thought to be a fixed value, representing the unchanging rate of expansion of the universe. However, recent studies have shown that it may not actually be constant and may change over time.

3. How is the Hubble Constant measured?

The Hubble Constant is typically measured using the distances and velocities of galaxies. By observing the redshift of light from distant galaxies, we can determine their velocities. Then, by measuring their distances using standard candles such as Type Ia supernovae, we can calculate the Hubble Constant.

4. Why is there debate about whether the Hubble Constant is really constant?

The debate about the constancy of the Hubble Constant stems from conflicting measurements using different methods. Some studies have found a value of around 70 km/s/Mpc, while others have found a value closer to 73 km/s/Mpc. This difference may indicate that the Hubble Constant is not truly constant, but rather varies over time.

5. What are the implications if the Hubble Constant is not constant?

If the Hubble Constant is not constant, it could have significant implications for our understanding of the universe. It may require a modification of the current standard model of cosmology and could potentially change our understanding of the age, size, and fate of the universe. Further research and observations are needed to fully understand the implications of a non-constant Hubble Constant.

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