Today we are doing a special episode on one of the largest problems in astronomy today: the expansion rate of the universe. This is going to be a story in three parts: An overview of how we know our universe is expanding, a look at the different factors that go into understanding that expansion rate, and today’s lack in conclusive measurements.




Today we are doing a special episode on one of the largest problems in astronomy today: the expansion rate of the universe. 

This is going to be a story in three parts: An overview of how we know our universe is expanding, a look at the different factors that go into understanding that expansion rate, and today’s lack in conclusive measurements.

Up until the early 1900s, our views on the evolution of the universe were largely based on natural philosophy. While the world’s dominant religions largely described the universe as being created in a singular creation event, many scientists and philosophers imagined the universe as an unchanging, static creation. The quandary came in trying to understand if the universe was static,  infinite in size, infinite in age, or some combination of all those things.

One of the earliest observational arguments was Heinrich Olbers ‘Dark night sky paradox”. In 1823, Olber clearly articulated a problem previously noted by the likes of Thomas Digges and even Johannes Kepler. This problem is, quite simply, that our sky is dark, and that in a universe of infinite size and infinite age everywhere you look in the sky should have you looking at a star, and that star’s light will have had time to reach us. Since the sky is actually dark, it has been argued that our universe must be finite in either age or size. If it’s finite in only age, than light from the most distant stars wouldn’t have had time to reach us. If it was instead finite in size, than there would be places you could look where the universe came to an end before you ever saw a star. The darkness at night doesn’t tell us how the universe is finite, only that it is finite. 

In this era without more subtle observables to help understand our universe, even Einstein tuned his models to assume our universe is static. In his 1917 formulation of the Cosmological considerations of the General Theory of Relativity, Einstein included a constant of integration whose value could be set in such a way that it would stop the universe’s expansion or contraction, and balance it into a static form. He assumed initially, this cosmological constant needed to have a value that made the universe unchanging. 

That same year, however, Vesto Slipher had measured the motions of all the nearby galaxies that were bright enough to be accessible to the telescope he was using at Lowell Observatory. He found that while a couple of the galaxies seemed to be moving toward us, the majority were moving away. In a generally static universe, he would have expected a random distribution of motions, with as many moving toward us as away. Hubble took these results, and then combined them with Henrietta Leavitt’s 1912 discovery that that Cepheid variables can be used to measure distances to nearby galaxies. By knowing from the Leavitt relationship where the galaxies are located, and by knowing from Slipher’s velocity measurements, and subsequent measurements he and Milton Humason acquired, Hubble was able to determine that the motions of galaxies are consistent with the over universe expanding. While our galaxy and nearby galaxies may be gravitationally interacting, and have motions that are related to the systems’ orbits, the motions of more distant galaxies are dominated by the expansion of the universe, with nearby objects appearing to move more slowly and more distant objects, that have more stuff to expand between us and them, appearing to move more quickly.

This idea was accepted fairly quickly, with Einstein changing his explanations for the terms in his cosmological applications of general relativity so that the equations necessitated an expanding universe. The question now became, how is it expanding? Was there a single Big Bang the created the universe from a single point? Or is our universe like some great sourdough starter, and it is simply bringing new matter into being so that every bit of universe is always expanding outwards. 

James Peebles at Princeton was one of the proponents of a Big Bang model, and his theoretical work predicted that if there was a Big Bang, we should see a wall of radiation in all directions that comes from the moment when our young universe cooled enough that atoms could form, and light, which had previously been trapped into a constant cycle of absorption and re-emission could instead finally fly free. Everywhere in the universe, light of a characteristic color would have been emitted, and everywhere we go, we should see a sphere of light from the part of the universe that is at the correct distance to be reaching us at this moment in time. 

And that wall of light, called the cosmic microwave background radiation, was found in 1964 by Robert Wilson and Arno Penzias. This was the ultimate case of one person’s signal is another person’s noise:  when they found the CMB, they were working to map on cosmic sources of microwave radiation that would interfere with ground based microwave communications.

Since then we’ve continued to find a myriad of other lines of evidence all showing that yes, the universe is expanding and started at a single moment of time with the Big Bang. The universe may or may not be finite in size, but we know with certainty that it is finite in age… we just don’t know what that age may be.

Early on, it was understood that our universe’s history and its fate are tied together by the rate of it’s expansion and the total mass density in the universe. For decades we discussed this as a problem balancing the expansion velocity against the essential drag created by the mass of the university trying to pull everything back together through gravity. Throughout most of my schooling, we imagined there were only 3 potential fates to the universe: It would eventually lose to gravity and collapse in on itself; it would in the fullness of time come to a stop, or it would simply keep expanding forever. 

To understand our fate we thought we needed to measure two things accurately: the mass-density of the universe and the expansion rate of the universe.

The universe had other plans however – in 1998, astronomers working to measure the expansion rate by looking at supernovae at a variety of distances discovered the universe is actually expanding apart at an accelerating rate, and this requires some additional something, something that has been given the terrible name Dark Energy, needs to be out there needing to be measured.

And it’s super hard to measure something that we don’t know what is.

As a starting point, observational cosmologists have been doing their best to measure as precisely as possible the apparently changing expansion rate of our universe, and all other measurables related to the mass and energy densities of the universe. If this is done right, we should find that each technique yields the same current age, and current expansion rate for our universe. 

And within error, all the techniques had been doing that until a couple years ago. As we got better at making different kinds of measurements, we suddenly found that we understand from looking at the modern universe doesn’t match. This is a problem. 

Since Vesto Slipher, astronomers have tried to understand our universe by looking at galaxy motions. Today, the best teams in the world build outwards, measuring velocities and distances, to get at a direct measurement of the expansion rate of the universe as a function of time. This isn’t easy, assumptions have to be made about the physics of Type Ia supernovae being constant over the age of the universe, and very tricky measurements of individual stars in nearby galaxies are required to calibrate those supernovae measurements. According to our best modern knowledge, the expansion rate of the universe based on these direct measurements is 73 km/s per megaparsec of space, with an error of 2.4% in that value.

In the modern era, we have also looked toward the cosmic microwave background for answers. The variations in color we see in images like this one from the Planck satellite indicate places where the early universe was a little bit more or less dense, and we can go from measuring these structures in the early universe, and based on assumptions about the geometry of the universe, we can calculate the age and expansion rate of the universe. Our best values from this technique give us a value of 67 km/s per megaparsec, again, with only a few percent error.

And these values of 73 and 67 do not overlap and imply we are missing some key piece of physics or there is something fundamentally wrong in our measurements.

The best way to try and deal with this kind of discrepancy is to find a tie breaking technique that will show which one of these results is correct by replicating it’s results in a completely new way. 

And this finally brings us to the new science that inspired this episode. On September 13, Science Magazine published a new study led by Inh Jee of the Max Planck Institute for Astrophysics. In this work looks at two gravitational lens systems and uses the geometry of these systems to study the geometry of the universe. 

Gravitational lens are just like their name implies: Gravity is acting like a lens to bend light to go in directions in otherwise wouldn’t go. This research two examined two different situations where a massive foreground galaxy bent the light of a background object so that it would come toward us instead of continuing in a straight line to some other part of the universe. These bent light paths are all longer than in distance then the direct path that background objects light could have taken, and the exact distance the light travels is determined both by the geometry of the system and the expansion rate of the universe, which is expanding the path length while the light is in transit. The cool thing about this technique is the background object’s size will appear to vary with path length, just like the size of a bus in your field of view will vary with the distance between you and the bus. A nearby bus or galaxy will both fill a lot of your field of view, while a distant galaxy or bus will only fill a small amount of your field of view. The team behind this study was able to use the angular size of the background galaxy to reconstruct the distances in the system. These measurements were then used to calibrate supernovae measurements and determine the expansion rate of the universe. This new technique landed on an expansion rate of 82.4 km/s/Mpc with an error of roughly 10%. This value, with this error, is consistent with the expansion rate measured using Supernova and cepheid variable stars to calibrate. While 10% error is more than we’d like, this new method is pointing toward an observational preference for the universe accelerating faster and thus being younger than cosmic microwave data would imply. More data is needed to confirm these results and hopefully reduce the error bars, but if these new techniques continues to reinforce the need for a higher expansion rate, and a younger universe… well, that also means we’re going to need to re-examine our understanding of the cosmic microwave background. Somewhere in this story is the hook that we can follow to new physics, the kind that could potentially get someone a Nobel Prize. 

It’s exciting to find something we don’t know, and while I wish it wasn’t something as fundamental as the age of the universe, at least the universe still appears to be older than the oldest stars.

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