# Part of the Hubble discrepancy can be explained by local underdensity

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In summary, the paper discussed in the conversation claims that there is observational evidence that we live in a region with slightly lower density than the universe average, which can explain a significant portion (5.5%) of the discrepancy in the Hubble constant between local measurements and the CMB measurement. However, this does not fully account for the discrepancy and other systematic errors may still need to be considered.
phyzguy
TL;DR Summary
The fact that we live in a local underdensity of matter (just by chance) can explain most of the discrepancy in the Hubble constant.
Interesting paper on the arXiv today. The authors claim that there is observational evidence that we live in a region with slightly lower density than the universe average, just by chance. Taking this into account can explain as much as 5.5% of the discrepancy in the Hubble constant between the local measurements and the CMB measurement. If you shift the local measurements to lower values by 5.5% (about 3.8 km/sec/Mpc), then the discrepancy is within the experimental errors.

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atyy and davenn
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Interesting article thanks for sharing, the article has an interesting argument.

Shouldn't this be part of the uncertainties already?
phyzguy said:
If you shift the local measurements to lower values by 5.5% (about 3.8 km/sec/Mpc), then the discrepancy is within the experimental errors.
The 5.5% are only for quite local measurements, not for the more distant supernova measurements.

mfb said:
Shouldn't this be part of the uncertainties already?
In an ideal world, sure. In practice it's difficult to account for this type of systematic error.

The basic way to understand this is that if we are in, say, a 2-##\sigma## underdensity, then it stands to reason that estimates which assume our local density is drawn from a random ensemble of possible local densities will be biased. If the underdensity was less severe, say 1-##\sigma##, then the random sampling of local densities should average to provide the correct result, since 1-##\sigma## is the expected deviation in such an ensemble.

If the magnitude of the local density can be determined and definitively shown to be below the average in the observable universe, then it would shift a lot of cosmological parameter measurements quite a bit. There'd probably be less dark energy, for instance. Such an underdensity might cause estimates of spatial curvature to be off. And yes, the Hubble constant would be different.

In the conferance videos that mfb shared ( about the Hubble discrepancy), Adam Riess stated that we do not live in a "giant" local void.

https://arxiv.org/abs/1810.03526

İf you want to listen start from the 19:00

http://online.kitp.ucsb.edu/online/enervac-c19/riess/

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atyy
phyzguy said:
Summary: The fact that we live in a local underdensity of matter (just by chance) can explain most of the discrepancy in the Hubble constant.

Interesting paper on the arXiv today. The authors claim that there is observational evidence that we live in a region with slightly lower density than the universe average, just by chance. Taking this into account can explain as much as 5.5% of the discrepancy in the Hubble constant between the local measurements and the CMB measurement. If you shift the local measurements to lower values by 5.5% (about 3.8 km/sec/Mpc), then the discrepancy is within the experimental errors.

I don't think 5.5% is enough to be consistent with experimental errors. The authors say "This is a differ-ence of about 9.6%, much larger than the combined error. The local estimates for the Hubble constant have been obtained well within a distance less than 100 Mpc, inside the region for which we found a significant underdensity in the matter distribution.Therefore our finding can at least explain part of the difference. But the discrepancy is larger than what could plausibly be accommodated by our observations. Thus one has to look in addition for other reasons for this discrepancy. There could wellbe further systematic effects which may have been overlooked or have been underestimated so far. "

atyy said:
I don't think 5.5% is enough to be consistent with experimental errors. The authors say "This is a differ-ence of about 9.6%, much larger than the combined error. The local estimates for the Hubble constant have been obtained well within a distance less than 100 Mpc, inside the region for which we found a significant underdensity in the matter distribution.Therefore our finding can at least explain part of the difference. But the discrepancy is larger than what could plausibly be accommodated by our observations. Thus one has to look in addition for other reasons for this discrepancy. There could wellbe further systematic effects which may have been overlooked or have been underestimated so far. "
What I meant was that if you remove the 5.5%, then the remaining discrepancy could just be statistical errors. The 9.6% discrepancy is claimed to be 4.4σ, if you believe all systematic errors have been accounted for ( for the record, I don't believe this). If 5.5% is due to the local underdensity, then the remaining 4.1% would just be just 1.9σ, which could just be statistical errors.

## 1. What is the Hubble discrepancy?

The Hubble discrepancy refers to the disagreement between the predicted expansion rate of the universe based on observations of the cosmic microwave background radiation and the observed expansion rate based on the distance and redshift of galaxies.

## 2. What is a local underdensity?

A local underdensity is a region of space that has a lower density of matter compared to the average density of the universe. This can occur due to the gravitational attraction of nearby massive structures pulling matter away from the region.

## 3. How does a local underdensity explain the Hubble discrepancy?

The presence of a local underdensity can cause the observed expansion rate of the universe to appear slower than the predicted rate. This is because the gravitational pull of the underdensity can counteract the expansion of the universe, making it appear as if the expansion rate is slower than it actually is.

## 4. Are there other factors that contribute to the Hubble discrepancy?

Yes, there are other factors that can contribute to the Hubble discrepancy, such as the presence of dark energy and the uncertainty in measuring the distances and redshifts of galaxies. However, the local underdensity is considered to be one of the main explanations for the discrepancy.

## 5. How do scientists study and measure local underdensities?

Scientists use various methods, such as galaxy surveys and gravitational lensing, to map the distribution of matter in the universe and identify regions of underdensity. They also use computer simulations to model the effects of underdensities on the expansion rate of the universe.

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