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.