The Crisis in Cosmology: We Don't Know How Big the Universe Is

What if we were to tell you that scientists have absolutely no idea how old the universe is? That may be a bit sensationalist as we do have some idea, but we don’t know for sure. More importantly, we don’t know why we don’t know.

The first reasonably accurate calculation for the age of the universe came in 1958 from astronomer Allan Sandage, derived from measuring the universe’s rate of expansion. There was a margin of error, but it was calculated that the universe was roughly 14.5 billion years old, give or take a billion years, with scientists believing the best estimate is 13.8 billion years. For nearly 50 years this age of the universe was accepted. As science improved, so too did our calculations and that margin of error became smaller.

Then in the year 2000, a new method for measuring the rate of expansion of the universe was created. At first, this was great. Because of the margin of error, both methods offered a possible range for the age of the universe, and those ranges overlapped. But as the years went on and our measurements became more precise, those ranges shrank in opposite directions, resulting in two different answers.

This discrepancy in the two different calculations is known as the Hubble tension, and it’s getting worse. Recent data from the Planck observatory has only further widened the gap between the two values. This means one of two things: either the math that has been double checked innumerable times is somehow wrong, or our current understanding of physics is even more incomplete than we realized, and there’s new physics out there for us to learn.

The Distance Ladder

To determine the age of the universe, we need to calculate the Hubble constant. This is the unit of measurement that describes the expansion of the universe, written in kilometers per second per megaparsec, and we promise that this all sounds more complicated than it is.

Basically, if we can measure how far away a star is and how fast it’s moving, we can figure out how long it took to get there. If you knew a car was traveling away from you at 60 mph and was 60 miles away, then you’d know it had been traveling for one hour. We’re just doing a similar calculation but with comically large numbers on a universal scale. Oh, and the math is much more complicated because of how the universe expands.

The Hubble constant is expressed in kilometers per second per megaparsec because the expansion of the universe is accelerating, so the further away from us something is the faster away from us it is traveling. This is all well and good, but how do we actually measure how far these objects are away from us? This is where we get the distance ladder, a series of observations and calculations that build upon one another to measure distances further and further away.

First, we begin with the local universe, local in this instance referring to objects within the Milky Way. If you hold your finger in front of your face and close one eye, then open that eye and close the other, you’ll notice that your finger moves from side to side relative to the background behind it. The closer your finger is to your face, the more it will move when you switch from one eye to the other.

This observed displacement of the object against a background is called parallax, and it happens because your two eyes are in different physical locations. Because the shift is greater the closer an object is, we can use this same principle to measure the distance from Earth to objects within the Milky Way. Just replace your finger in our demonstration with some distant star and replace your eyes with the view from Earth at opposite ends of its orbit around the sun, a distance of approximately 300 million km apart. By measuring the shift from those two locations, we can calculate how far away it is.

Because the parallax of an object gets smaller the further away it is to the point of being imperceptible even with advanced technology, a new system needed to be designed to measure the distance of objects even further away than our galaxy. This brings us to a special type of star known as a Cepheid Variable. Cepheids are a type of bright, pulsating star, and in 1908 American astronomer Henrietta Leavitt discovered a direct correlation between how bright a Cepheid was and how fast it was pulsating.

Because these two values were directly proportional, we could measure the apparent brightness and the pulsation period of a Cepheid that was tens of millions of light years away. The Cepheid’s period would tell us what the star’s actual brightness was, and its distance could be determined by comparing that to the apparent brightness.

This made Cepheids invaluable for their use as “standard candles”. Stars come in all different shapes, sizes, and brightness, but a standard candle is an object whose luminosity is always known, in this case as a function of its pulsation period. There was just one other step before scientists could begin using these candles to measure even greater distances across the universe.

First, they had to use parallax to measure the distance of Cepheids within the Milky Way. The relationship between the period and brightness of a Cepheid was known, but they needed the parallax measurements of nearby Cepheids to create a numerical baseline that they could use to extend this investigation to even greater distances.

Unfortunately, Cepheids become too faint for us to see after about 100 million light years. That may seem really far, but it’s only about 0.1% of the width of the universe, so we were going to need another technique. That brings us to the final rung on our ladder: type 1A supernovas.

A type 1A supernova occurs in binary star systems where at least one of the two stars is a white dwarf. Because there is a fixed critical mass at which a white dwarf will go supernova, all type 1A supernovas should have the same peak level of brightness. The problem is that while we know this is true, we haven’t seen one of these supernovas up close so we didn’t have any way to know what that peak brightness level actually was.

To solve this problem, we needed to find galaxies in which a 1A supernova went off that also contained Cepheids. Using the previous method, we could calculate the distance to that galaxy using the Cepheids. Once we knew how far away that galaxy was, we could use the distance and the apparent luminosity of the supernova to determine the actual luminosity of these supernovas.

With that value in hand, we could then locate 1A supernovas that occurred in galaxies much farther than we could see Cepheids and use the apparent brightness and actual brightness to calculate its distance away. Because these supernovas are so massively bright, this allowed us to measure the distance of objects up to 1 billion light years away.

The first real consensus reached using this method came in 2001, when the Hubble constant was measured at 72, plus or minus eight. Our measurements have become much more precise over the past two decades, and the most recent calculations using a distance ladder in February of 2022 gave a constant of 73.4 plus or minus about one. That is a much smaller range, and therein lies the problem.

Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is the first light created in the universe, originating from when the universe was only 370,000 years old. Before this, the universe was too hot for atoms to form leaving it essentially opaque. When the temperature cooled enough for the first hydrogen and helium atoms to form, the universe became transparent and light could be emitted.

The CMB is only detectable in long, radio wavelengths of light that reaches us from every direction. The temperature of the universe at the time this light was first emitted was imprinted on the CMB, and at first it looked like it was completely uniform with a temperature of just under three Kelvin. Now that we have much more sophisticated equipment, we see that the CMB actually contains tiny temperature variations.

The darker blue areas on images of the CMB represent the coldest temperatures while the red represents the hottest temperatures. Those colder regions show the areas that would have had a high enough density of the newly formed atoms to build the first stars. Essentially, the CMB is a 13.7 billion year old blueprint for the construction of the entire universe.

To give you an idea of just how incredible it is that we can even detect and decipher this blueprint, the difference in temperature between the coldest and hottest points in the CMB is only 200 microKelvin, or 0.0002 degrees Celsius. It was also discovered entirely by accident.

Everything within this blueprint can be described using six parameters: the ratios of matter, dark matter, and dark energy, the shape and structure of the universe, and the rate of expansion of the universe. When we compare the CMB to our best model of the universe, the data lines up almost perfectly. Since we already have values for five of the six parameters, we can use all of this to calculate a value for the rate of expansion of the universe, the Hubble constant.

While measurements using the distance ladder date back to the 1950s, using CMB to calculate the Hubble constant is a much newer method. The first such calculations came in 2003 using data from the then new Wilkinson Microwave Anisotropy Probe. They measured the Hubble constant at 72, plus or minus 5, perfectly in line with contemporary values given from the distance ladder. A few years later, the Planck space observatory was launched to take measurements of the CMB with a much higher degree of sensitivity.

Planck was decommissioned in October of 2013, but it wasn’t until 2018 that the final results from the team were published. That result was a Hubble constant of 67.6 plus or minus 0.4. This no longer matched with the distance ladder’s newer result of 73.4, and that discrepancy, the Hubble tension, is highly statistically significant.

The Implications

So what exactly does all of this mean? Because of the Hubble tension, the age of the universe is uncertain. The ages calculated by those two values differ by about a billion years, which is over 7% of the entire age of the universe. And we don’t know what’s causing it.

The first possibility is that something within the distance ladder is incorrect. Since each step builds upon the previous one, any mistake would then propagate through the rest of the results. Even if our calculations involving Cepheids and type 1A supernovas are correct, if the initial measurements using parallax were wrong it would cause everything else to be wrong as well.

This means that if there is a problem with the ladder, there’s no way to easily determine where the mistake lies and every part of it needs to be measured and calculated over and over again. It could also be that the problem isn’t with the calculations themselves, but that something about the method itself is fundamentally wrong.

The much more exciting possibility is that the calculations using CMB are wrong. This would mean that something about our best current model of the universe is either incorrect or incomplete. There are a lot of theories being thrown around about what could be causing this difference, but thus far nothing that has been tested.

Currently, it seems more likely that the issue lies within the distance ladder, specifically with the Cepheid variables. It was speculated that the Cepheid measurements were “biased bright”, meaning that we were measuring them as being brighter than they actually are. Because we can only take images with limited resolution at massive distances, it’s possible the light from a Cepheid and another nearby star were essentially overlapping, making it look like a single star with a higher apparent brightness than it should have had.

This theory was proposed when scientists decided to try to use red giant stars as a standard candle instead of Cepheids for the second rung of the distance ladder. Using this method, they calculated a value for the Hubble constant that was no longer significantly different from the one found using CMB.

Wrap Up

Our understanding of the universe is constantly improving, but as we learn more we often find things that we previously believed to be true were wrong. It was previously believed that the Earth was the center of the solar system and that the Milky Way Galaxy was the entire universe. Even the idea that Earth could be millions or even billions of years old is relatively new. These new discoveries are exciting because they give us greater knowledge about the universe.

If it turns out that our distance ladder was incorrect, we’ll probably know very soon. One of the missions for the James Webb Space Telescope’s first year is to examine Cepheids that were previously imaged by the Hubble telescope. Because of the incredible increase in resolution provided by JWST, we should be able to determine whether the Cepheids really were biased bright or if the cause of the Hubble tension lies somewhere else.

There already is some preliminary data from the JWST that has been used to check this hypothesis. While the initial results don’t show any difference from the measurements taken using Hubble, the data being collected was calibrated for an unrelated mission and so it’s not necessarily indicative of what the final determination will be.

We can only hope that once JWST completes this mission, it reveals that our distance ladder has been correct all along and it is the Hubble constant derived from the CMB that was incorrect. If that is the case, hopefully it will result in the discovery of lots of new and exciting discoveries in physics.

Key Takeaways

• The universe’s age is uncertain due to differing measurements from two methods.
• The Hubble constant, crucial for age determination, is calculated via the distance ladder and cosmic microwave background.
• The Hubble tension arises from discrepancies between the two Hubble constant measurements.
• The James Webb Space Telescope may resolve the Hubble tension by examining Cepheid variables.
• New physics may be needed to explain the Hubble tension and refine our universe model.

Sources

• Original Side Projects video: The Crisis in Cosmology: We Don’t Know How Big the Universe Is
• Hero image source by Church of emacs / openverse, by-sa.

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