I used to think the question, “how much does the universe weigh?” had an easy answer:
It weighs everything.
But it turns out my glib deduction is not the one scientists have been looking for,
and they’ve been probing the cosmos searching for a quantifiable answer.
However, the results they’ve come back with are inconsistent depending on how they measure it,
and it’s causing a bit of tension that may rewrite the Standard Model of Cosmology
Asking how much the universe “weighs” isn’t really a question that makes sense.
After all, weight is calculated by multiplying an object’s mass by the acceleration due to gravity—
so how can you “weigh” an object that’s floating in space?
A better approach might be to ask what is the total mass and energy of the universe,
but that would be an incomprehensibly huge number.
Here’s what it looks like anyway.
So, to make conceptualizing the mass of the universe a bit easier,
let’s break it down and try to figure out the density of matter in the universe,
i.e. how much stuff there is on average in a volume of space.
That measurement, along with the degree to which stuff in the universe has clumped together,
is what’s known as sigma-eight.
But there’s a growing concern: two different approaches to measuring sigma-eight
have come back with two different numbers.
One method uses weak gravitational lensing.
You may be familiar with the concept of gravitational lensing,
which usually comes up when we talk about black holes.
It occurs when the gravitational pull from massive objects warp space and bend the path light takes,
distorting the image.
With black holes this distortion can be quite pronounced.
But matter and energy distributed throughout the universe bends light too, just almost imperceptibly.
So, to spot this weak-lensing effect,
astronomers observed millions of galaxies across about 350 square degrees of the sky.
The logic is that all these galaxies should be randomly oriented towards us,
so if nothing bends their image, the average shape should be nearly circular.
But thanks to matter and energy bending light’s path in the space between the galaxies and us,
their shapes average out to be just slightly elliptical.
Taking that into account, along with the distances to the galaxies,
astronomers were able to get a figure for sigma-eight.
By their calculations it comes out to be about 0.74, in case you were wondering.
But another group of astronomers took a different approach,
one that uses the cosmic microwave background, or CMB,
which is the earliest light we can see from the start of the universe.
Using observations taken by ESA’s Planck Satellite,
scientists mapped the temperature and polarization of the CMB,
then ran the clock forward to today, taking into account what we think the universe is made up of.
This ratio of ordinary matter to dark matter to dark energy in the universe
is what’s known as the standard model of Cosmology.
Based on their data and assumptions, their calculations for sigma-eight came back with a higher value, 0.81.
That difference has been dubbed the sigma-eight tension,
and to longtime viewers of this channel the problem may sound vaguely familiar.
A similar tension has arisen around the Hubble Constant,
or the rate at which the universe is expanding.
Calculations of the Hubble constant using a variety of methods,
including gravitational lensing and the map of the CMB,
have come back with different results that are statistically significant.
There’s a 1 in 3.5 million chance the discrepancy between calculations using the CMB
and those using gravitational lensing is a fluke,
and this so-called Hubble tension has left astronomers and cosmologists in a state of crisis.
Whether or not the sigma-eight tension turns out to be a crisis of the same magnitude remains to be seen.
Right now, there’s only a 1 in 100 chance the mismatched results are due to a statistical quirk.
And it’s possible one or both methods of finding sigma-eight had errors that would explain the difference.
Maybe the way the astronomers measured the distance to their millions of warped galaxies, which they acknowledge was less precise but more efficient, threw their numbers off.
Or maybe our estimates of the universe's makeup are wrong.
These results, along with the debate over the Hubble constant,
could lead us to revise the standard model of cosmology.
In the meantime, if somebody asks you how much the universe weighs,
just go ahead and say “everything.”
No matter what the figure for sigma-eight really is, the universe as a whole is not very dense.
By one estimate, on average a cubic meter of the universe
contains the mass equivalent of just under 6 protons.
If you want to know more about the Hubble tension and hear a lot of puntastic scientific acronyms,
check out my video on it here.
Don’t forget to subscribe, and thanks so much for watching. I’ll see you next time on Seeker.