# Voids and the Hubble Constant

• I
Gold Member
It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have measured the Hubble constant lower. Since when the light passes an underdense region it gets less redshifted. Less redshift means less expansion or lower Hubble constant right? But we are measuring it higher. So shouldn't we live in a more dense region rather than an underdense region, to explain the discrepancy? What am I missing here?

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2022 Award
It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have measured the Hubble constant lower. Since when the light passes an underdense region it gets less redshifted. Less redshift means less expansion or lower Hubble constant right? But we are measuring it higher. So shouldn't we live in a more dense region rather than an underdense region, to explain the discrepancy? What am I missing here?
I think what you are missing is that areas are either expanding or are not expanding so there either IS redshift or there is NOT redshift, not more redshift or less redshift . Relatively dense areas such as galactic clusters are not expanding and areas between galactic cluster are expanding. In measuring red shifts of distant galaxies, this has no effect since most areas on galactic scales are low density and anyway it all averages out.

Also, your statement "Since when the light passes an underdense region it gets less redshifted" is backwards.

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Since when the light passes an underdense region it gets less redshifted. Less redshift means less expansion or lower Hubble constant right? But we are measuring it higher. So shouldn't we live in a more dense region rather than an underdense region, to explain the discrepancy? What am I missing here?
I think you are missing that if light travels into an underdense region it arrives there with a tiny additional gravitational redshift, because light looses energy if it escapes from an overdense region.

Its the other way round if light enters an overdense region like a supercluster of galaxies. First it it gets blueshifted and then, leaving the cluster it gets redshifted. In this case the net effect is a slight blueshift corresponding to the participation of the supercluster in the expansion of the universe.

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with a tiny additional gravitational redshift,
why this the case ? I mean why they are not cancel each other out and become 0 ?

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I think what you are missing is that areas are either expanding or are not expanding so there either IS redshift or there is NOT redshift, not more redshift or less redshift . Relatively dense areas such as galactic clusters are not expanding and areas between galactic cluster are expanding. In measuring red shifts of distant galaxies, this has no effect since most areas on galactic scales are low density and anyway it all averages out.
I did not quite understand. You mean it does not matter where we are located ?

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I did not quite understand. You mean it does not matter where we are located ?
In measuring the redshift of distant galaxies, yes that's what I mean. That is consistent with the Cosmological Principle.

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why this the case ? I mean why they are not cancel each other out and become 0 ?
Why should they cancel? If we are in a giant void we measure photons coming from outside the void direction-independent slightly redshifted compared to a measurement in a region where we are not in a void.

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Why should they cancel? If we are in a giant void we measure photons coming from outside the void direction-independent slightly redshifted compared to a measurement in a region where we are not in a void.
I think his point is this: if a photon is coming into a galactic cluster it gets a slight gravitational boost (blue shifting) due to the gravity of the cluster as it comes in. As it goes out, it is retarded (red shifted) by an equal amount, so why would those not be equal amounts?

Arman777
Gold Member
I think his point is this: if a photon is coming into a galactic cluster it gets a slight gravitational boost (blue shifting) due to the gravity of the cluster as it comes in. As it goes out, it is retarded (red shifted) by an equal amount, so why would those not be equal amounts?
Yes exactly

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Yes exactly
Thanks @phinds for communication.
Well but I mentioned "a tiny additional gravitational redshift " in post #3 regarding the void scenario.

In the supercluster scenario there results a very tiny blueshift as net effect (after the photon has left the cluster) as mentioned. Within the cluster we would measure a blueshift instead of a redshift within a void. This was just a counterexample but unfortunately caused some confusion.

@Arman777 can you please clarify to which scenario your question in post #4 refers to?

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Well let's have a normal dense region. Like a normal potential well then we enter a void which is an underdense region. So at first the redshift increases by an amount of ##\Delta z##. And when its coming out of the void the light gets blueshifted by the same amount ##\Delta z##. So I thought the net effect should be zero.

However I guess when the light is coming towards us its getting redshifted indeed but its not coming out. So it will stay as redshifted.

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Well let's have a normal dense region. Like a normal potential well then we enter a void which is an underdense region. So at first the redshift increases by an amount of ##\Delta z##. And when its coming out of the void the light gets blueshifted by the same amount ##\Delta z##. So I thought the net effect should be zero.

However I guess when the light is coming towards us its getting redshifted indeed but its not coming out. So it will stay as redshifted.
Yes within the void we measure it redshifted. If it "gets blueshifted by the same amount ##\Delta z##" coming out depends on the influence of the expansion of the universe on the void. If the void grows in size during the photon is passing through then its not the same amount ##\Delta z## but a little less, so the net effect is a tiny redshift. In other words it gains less energy coming out than it looses coming in.

Arman777