Planck and Cepheids: new problems?


Some of the biggest news in astronomy and cosmology is the recent announcements of results from the Planck mission that observed the cosmic microwave background. One of the key results is that the universe is actually a bit older than previously estimated, now 13.82 billion years old as opposed to 13.73 billion years old. What’s 100 million years difference matter in the grand scheme?

Well it is not the age that is the challenge but the expansion rate of the universe or what is called the Hubble Constant. The universe is known to be expanding, for instance when we look at far away galaxies we observe them to be moving away from us and galaxies twice as far are moving twice as fast. This rate tells us about the age of the universe and how much mass and energy is in the universe. The Planck mission found the rate to be 67.3 km/s/Mpc (1 Mpc (megaparsec) = 100000 parsec = 3,260,000 light years). This is interesting, the predecessor to Planck, the WMAP mission measured about 72 km/s/Mpc while other methods measured about 72 - 75 km/s/Mpc.

It is surprising that Planck is so much lower and raises a few questions about these other methods. The best other method employed so far requires measuring the velocity of far off galaxies using their spectra and searching for standard candles in those galaxies to measure distances. When an object is moving and emits light, light at various wavelengths will be shifted, an object moving away appears redder than it would if it were not moving. By measuring how much the light is shifted then we can measure the speed of a galaxy. This is analogous to how the sound of a train changes when it is traveling towards and away from an observer.

Measuring the distances to these galaxies requires finding standard candles. A standard candle is an object that, when observed, allows astronomers to measure the distance to that object. The most commonly employed standard candle is a Cepheid, a personal favourite of mine that I have studied since my PhD. A Cepheid is a variable star whose brightness oscillates for a few days up to 200 days, and that period of oscillation is a direct measure of how much light a Cepheid is emitting. By observing how bright a Cepheid appears and its period, hence its actual brightness, we can measure the distance to that star and the galaxy in which it lives. It is these objects that have been primarily used to measure the Hubble Constant and the best
measurements suggest a greater rate of expansion than the new Planck results.

What does this mean? Could Planck be in error? Or Is there something we are missing in understanding these favoured standard candle? Perhaps Planck is telling us that we need to better understand these stars and that we need to revisit them in greater detail. This is difficult challenge and I am stymied how the Cepheid measurements might be so off. It is important to figure this out because Cepheids will provide an independent check of the Planck results when the James Webb Space Telescope comes online and astronomers will be able to observe Cepheids at unprecedented distances with unprecedented accuracy.

The newly-released Planck results are a giant leap forward in the understanding of the beginning of the universe and the results have consequences throughout the field of astronomy, even to the seemingly unrelated field of stellar physics. But Planck might just be requiring us to rethink what we know about these stars. For every answer, more questions arise; this is why science is awesome.
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Naples and the Distance to Everything

After I returned to Germany from China, I turned right back around and hopped on a plane to Naples for a conference on the Cosmic Distance Scale. The goal of this conference was to understand how well astronomers can measure the distances to far away stellar objects and galaxies and use this information to measure the rate of expansion of the universe, denoted by Hubble’s Constant H0. By more precisely measuring the expansion, it is hoped that we can better understand the structure of the universe, and the physics of the universe such as dark energy.

I contributed a small poster focusing on the details of Cepheids, how they lose mass in winds, and how this mass loss contributes to the amount of infrared light (light at wavelengths longer than that which can be seen by people). Cepheids are important distance indicators because they pulsate. The amount of light a Cepheid emits varies over a few - 100 days and repeats over and over again, and the period of this variation tells us how much light this is, not just the amount of light we see. Thus, with this information we can measure the distances to these stars and to the galaxies that we see Cepheids in. However, they are not perfect distance indicators, the relation between the amount of light emitted and the period of variation also depends on what a Cepheid is composed of. For example, the Sun is about 70% hydrogen, 28% helium, and 2% everything else and Cepheids is our galaxy have very similar compositions. In another galaxy though a Cepheid that emits the exact same amount of light as our Galactic Cepheid may have composition 73%, 26% helium and 1%, but the period of variation will be different. This means we would predict a wrong distance to that galaxy, and means we have an uncertainty in measuring distance and H0.

In infrared light, it is believed that this uncertainty due to composition decreases and thus one can measure distances more precisely. For my poster, I showed that Cepheids have winds that generate an infrared excess due to dust forming around the Cepheid. The amount of dust and hence amount of extra infrared light depends on the composition of the Cepheid and means that the brightness we see is larger than it would be if there were no dust. Since we don’t know how much dust may be around a Cepheid in another galaxy then there is an extra uncertainty in measuring distances that is not understood. It was not the most positive message for measuring cosmic distances but is interesting for understanding how these stars evolve.

There were some very interesting talks, discussing a wide variety of objects for measuring distances. There were talks on supernovae, planetary nebulae, RR Lyrae stars, and more. The talk on supernovae was interesting in that the researchers used Cepheids to calibrate the distance to nearby explosions of white dwarf stars. An exploding white dwarf emits almost the exact same amount of light as every other exploding white dwarf, (though there are exceptions) which makes it another very powerful distance indicator, and you can see further way than almost every other distance indicator. Using these stars, the researcher and his team determined one of the most precise values of the Hubble Constant ever, it is almost precise enough to constrain other aspects of cosmology, such as the amount of matter in the universe.

One lesson I took from the conference is that these great results depend on who and how the observations are treated. Using the same Cepheids, different researchers found a very different value of the Hubble Constant. This tells me that astronomers must be very careful and there needs to be multiple tests to understand measurements like the Hubble Constant and we need to be careful in taking these great measurements and announcements too seriously. The results may easily change.

Beyond the conference, Naples was an interesting city. There was a dramatic juxtaposition between the amazing view of the Mediterranean Sea and surrounding mountains and views of garbage piling up on street sides and building looking old and run down. This was a little disappointing but the food, the food, more than made up for it. The food in Naples was amazing. However, the highlight of the trip was a visit to Pompeii, which is worth the trip all on its own. I’ll talk about that in a future post.
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