# Why Does Light Lose Energy with Universe Expansion?

• sancharsharma
In summary, the physical reason for light losing energy as the universe expands is due to the stretching of light waves caused by the expansion of space. This is a valid explanation, supported by the mathematics of general relativity, and can be described as both cosmological Doppler shifts and kinematic Doppler shifts.
sancharsharma
Can anyone please explain the physical reason of why is light losing energy as universe is expanding?

Can you give a reference to that statement?

sancharsharma said:
Can anyone please explain the physical reason of why is light losing energy as universe is expanding?
A simple way to look at is that the expansion is stretching the light waves. Longer wavelength is equivalent to lower energy.

@mathman,
But the wavelength of light is not merely a distance between two points. As a support it doesn't follow lorentz length contraction. It is distance per cycle.
So are you sure that it is a valid argument to say that wavelength expands as universe expands?

sancharsharma said:
@mathman,
But the wavelength of light is not merely a distance between two points. As a support it doesn't follow lorentz length contraction. It is distance per cycle.

Cosmological expansion isn't the same thing as Lorentz contraction.

sancharsharma said:
So are you sure that it is a valid argument to say that wavelength expands as universe expands?

It is valid to describe cosmological Doppler shifts verbally as kinematic Doppler shifts, and it is also valid to describe them verbally as being due to the expansion of space. These are just two different ways of talking about the mathematics of general relativity.

## 1. Why does light lose energy with universe expansion?

Light loses energy with universe expansion because the expansion of space causes the wavelength of light to increase. As the wavelength increases, the frequency decreases, and since energy is directly proportional to frequency, the energy of the light decreases as well.

## 2. How does the expansion of space affect light?

The expansion of space affects light by increasing the distance between objects, which causes the wavelength of light to stretch. This stretching of the wavelength results in a decrease in energy, known as redshift.

## 3. Does all light lose energy with universe expansion?

Yes, all light loses energy with universe expansion. This phenomenon, known as cosmological redshift, affects all wavelengths of light, including visible light, infrared, and radio waves.

## 4. Is the energy lost by light with universe expansion significant?

The energy lost by light with universe expansion is relatively small. The amount of energy lost depends on the distance the light has traveled and the rate of expansion. In most cases, the energy loss is negligible compared to the overall energy of the light.

## 5. Can light regain its lost energy in a expanding universe?

No, light cannot regain its lost energy in an expanding universe. The energy loss due to redshift is a result of the expansion of space, which continues to stretch the wavelength of light. Therefore, the energy lost by light is permanent in an expanding universe.

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