Paul M. Sutter, an astrophysicist from SUNY Stony Brook is host of "Ask a Spaceman", "Space Radio," "How to Die In Space" and is also the author of "How to Die In Space." This article was contributed by Sutter to Space.com's Expert voices: Op-Ed and Insights.
In 2014, measurement of the Hubble constant began to differ from the current-day expansion rate. The distance measurements were approximately 10% higher than the ones taken from the near universe. Although it may not sound like much (and it doesn't considering the immense feats of science required to make these measurements), there were only about 22% uncertainties in those measurements.
Statistics are worth looking into if there is a 10% difference and a 2% uncertainty. Over 300 solutions have been proposed to the "crisis of cosmology" since 2014. Cosmologists are not able to agree on all of these solutions, and the crisis continues to worsen.
Related: Is there really an 'crisis in cosmology'?
The LCDM model
Our modern understanding of the universe's history is at stake. This model is often abbreviated LCDM. The model relies on a few assumptions as any other science model. It assumes general relativity holds at all scales, and that the universe is homogenous (the same in all directions). It assumes that the universe is flat, and that dark matter interacts with normal matter less often than normal. That's the "CDM" portion, which stands for cold dark matter. It also assumes that dark energy, or the "Lambda", is another substance that keeps the universe expanding at a constant density.
The LCDM only has six parameters after these assumptions have been established (which they do, based upon numerous past observations). To get these numbers, you will need to take cosmological measurements. However, once you have done that, the model can then predict all aspects of the universe up to the current-day expansion rate.
The cosmic microwave background (CMB) is a great place to determine the values of six parameters. It is the remnants of 380,000 year ago when the universe was just 380,000 years old. Because it is large, easy to measure, and simple to comprehend, the CMB is very useful.
You can use measurements of the CMB from European Space Agency's Planck satellite missions to fill in the gaps in the LCDM model, and you will have a good grasp on the entire history of our universe.
From the stars to the ladders
Here comes the tension. The measurements of the early universe provide a wealth of information about the parameters of the LCDM model. These measurements are not just from the CMB, but also from the so-called baryon audio oscillations. They show subtle shifts in galaxy position left over from giant sound waves that crashed into the early universe.
You can predict a Hubble constant value around 68 km/s/Mpc regardless of what combination of early-universe measurements are used to fill out the LCDM model.
Okay, great. It's done, right? But not so quickly.
It is also possible to directly measure Hubble constant. This requires you to measure the speeds and distances of several objects. There are many options available, including Type Ia Supernovas, Galaxy Properties, Mira Variables and certain types of red giant star.
Except for the red-giant technique, all local measurements of Hubble constant show a much higher number like 74 km/s/Mpc.
The red-giant method, which can be frustrating or interesting, gives a number that is right between the extremes, hence the crisis.
There is no other way.
There are two ways to approach the Hubble constant. Both approaches are well-tested, well-studied and well understood. The LCDM model is extremely successful at describing and forecasting a variety of cosmological observations. The CMB measurements are by far the most precise measurements ever made in the history of astronomy.
The supernova measurements, on the other hand are legitimate. Different probes of Hubble's constant also give comparable results.
These two views of the universe shouldn't be able to agree. But we don't have a common value for the Hubble constant.
This "crisis", which is so important to cosmologists, is the first significant occurrence in cosmology since dark energy was discovered over 20 years ago. Nature whispers to us when measurements don't agree. This is a great place to start learning more about the cosmos.
There have been more than 300 solutions proposed to the cosmology crises. Some advocate additional physics during the CMB era. Some argue for dark energy to do something unusual in the past. Some alter physics fundamentally, causing supernova observations to be distorted.
However, there is no one solution that can explain the vast amount of evidence in cosmology. We are far from a consensus.
Personally, I believe that "if it is interesting, it is probably wrong." The most boring explanation is that we are misinterpreting something about measuring the Hubble constant on local scales.
Only time will tell.
Listen to "Ask A Spaceman", a podcast that explains the subject. It is available on iTunes and askaspaceman.com. Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
