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--------------- 2771 - HUBBLE CONSTANT - how fast is universe expanding?
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- Scientists are considering whether they may need to come up with a new model for the underlying physics of the universe in order to explain the expansion rate.
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- The “Hubble constant rate of expansion” also sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves.
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- There is compelling reason to believe that there is something fundamentally flawed in our current model. A recent measurement of the Hubble constant uses a kind of star known as a red giant. These new observations, made using Hubble Space Telescope, indicate that the expansion rate for the nearby universe is just under 70 kilometers per second per megaparsec ( 70 km/sec/Mpc).
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- One parsec is equivalent to 3.26 light-years distance. And, put in more familiar units the expansion rate is 49,300 miles per hour per every million light years distance.
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- This new measurement is slightly smaller than the value of 74 km/sec/Mpc recently reported using Cepheid variable stars, which are stars that pulse at regular intervals that correspond to their peak brightness.
- A central challenge in measuring the universe's expansion rate is that it is very difficult to accurately calculate distances to distant objects.
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- In 2001 the measured value using Cepheid variables was a Hubble constant of 72 km/sec/Mpc.
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- In 2019. scientists took a very different approach: building a model based on the rippling structure of light left over from the big bang, which is called the Cosmic Microwave Background.
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- These measurements allowed scientists to predict how the early universe would likely have evolved into the expansion rate astronomers can measure today. Scientists calculated a value of 67.4 km/sec/Mpc, in significant disagreement with the rate of 74.0 km/sec/Mpc measured with Cepheid stars at that time.
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- Astronomers have looked for anything that might be causing the mismatch. They have sought to check their results by establishing a new and entirely independent path to the Hubble constant using an entirely different kind of star.
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- Certain stars end their lives as a very luminous kind of star called a red giant, a stage of evolution that our own Sun will experience billions of years from now. At a certain point, the star undergoes a catastrophic event called a helium flash, in which the temperature rises to about 100 million degrees and the structure of the star is rearranged, which ultimately dramatically decreases its luminosity. Astronomers can measure the apparent brightness of the red giant stars at this stage in different galaxies, and they can use this as a way to tell their distance.
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- The Hubble constant is calculated by comparing distance values to the apparent recessional velocity of the target galaxies, how fast galaxies seem to be moving away. These calculations give a Hubble constant of 69.8 km/sec/Mpc.
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- NASA's upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST), scheduled to launch in the mid 2020s, will enable astronomers to better explore the value of the Hubble constant across cosmic time.
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- WFIRST, with its Hubble-like resolution and 100 times greater view of the sky, will provide a wealth of new Type 1a supernovae, Cepheid variables, and red giant stars to fundamentally improve distance measurements to galaxies near and far.
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- The problem is that the new measurements exacerbate a discrepancy between previously measured values of the Hubble Constant and the value predicted by the model when applied to measurements of the cosmic microwave background. They find that galaxies are nearer than predicted by the standard model of cosmology. The debate is over whether this problem lies in the model itself or in the measurements used to test it.
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- Edwin Hubble, after whom the orbiting Hubble Space Telescope is named, first calculated the expansion rate of the universe (the Hubble Constant) in 1929 by measuring the distances to galaxies and their recession speeds. The more distant a galaxy is, the greater its recession speed from Earth.
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- Today, the Hubble Constant remains a fundamental property of observational cosmology and a focus of many modern studies.
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- Measuring recession speeds of galaxies is relatively straightforward. Determining cosmic distances, however, has been a difficult task for astronomers. For objects in our own Milky Way Galaxy, astronomers can get distances by measuring the apparent shift in the object's position when viewed from opposite sides of Earth's orbit around the Sun, an effect called parallax. The first such measurement of a star's parallax distance came in 1838.
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- Beyond our own Galaxy, parallaxes are too small an angle to measure, so astronomers have relied on objects called "standard candles," so named because their intrinsic brightness is presumed to be known. The distance to an object of known brightness can be calculated based on how dim the object appears from Earth. These standard candles include a class of stars called Cepheid variables and a specific type of stellar explosion called a Type 1a supernova.
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- Still another method of estimating the expansion rate involves observing distant quasars whose light is bent by the gravitational effect of a foreground galaxy into multiple images. When the quasar varies in brightness, the change appears in the different images at different times. Measuring this time difference, along with calculations of the geometry of the light-bending, yields an estimate of the expansion rate.
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- Determinations of the Hubble Constant based on the standard candles and the gravitationally lensed quasars have produced figures of between 73 and 74 kilometers per second per megaparsec.
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- Measurements of the cosmic microwave background, the leftover radiation from the Big Bang, produce a value of 67.4, a significant and troubling difference. This difference, which astronomers say is beyond the experimental errors in the observations, has serious implications for the standard model.
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- The model is called Lambda Cold Dark Matter, where "Lambda" refers to Einstein's cosmological constant and is a representation of dark energy. The model divides the composition of the Universe mainly between ordinary matter, dark matter, and dark energy, and describes how the Universe has evolved since the Big Bang.
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- The “Megamaser Cosmology Project” focuses on galaxies with disks of water-bearing molecular gas orbiting supermassive black holes at the galaxies' centers. If the orbiting disk is seen nearly edge-on from Earth, bright spots of radio emission, called masers, radio analogs to visible-light lasers, can be used to determine both the physical size of the disk and its angular extent, and therefore, through geometry, its distance. The project's team uses the worldwide collection of radio telescopes to make the precision measurements required for this technique.
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- In their latest work, the team refined their distance measurements to four galaxies, at distances ranging from 168 million light-years to 431 million light-years. Combined with previous distance measurements of two other galaxies, their calculations produced a value for the Hubble Constant of 73.9 kilometers per second per megaparsec.
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- Testing the standard model of cosmology is a really challenging problem that requires the best-ever measurements of the Hubble Constant. The discrepancy between the predicted and measured values of the Hubble Constant points to one of the most fundamental problems in all of physics, so we would like to have multiple, independent measurements that corroborate the problem and test the model.
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- Our method is geometric, and completely independent of all others, and it reinforces the discrepancy. The maser method of measuring the expansion rate of the universe is elegant, and, unlike the others, based on geometry.
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- By measuring extremely precise positions and dynamics of maser spots in the accretion disk surrounding a distant black hole, we can determine the distance to the host galaxies and then the expansion rate. Our result from this unique technique strengthens the case for a key problem in observational cosmology.
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- Their measurement of the Hubble Constant is very close to other recent measurements, and statistically very different from the predictions based on the CMB and the standard cosmological model. All indications are that the standard model needs revision.
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- Astronomers have various ways to adjust the model to resolve the discrepancy. Some of these include changing presumptions about the nature of dark energy, moving away from Einstein's cosmological constant.
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- Others look at fundamental changes in particle physics, such as changing the numbers or types of neutrinos or the possibilities of interactions among them. There are other possibilities, even more exotic, and at the moment scientists have no clear evidence for discriminating among them.
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- This is a classic case of the interplay between observation and theory. The Lambda CDM model has worked quite well for years, but now observations clearly are pointing to a problem that needs to be solved. Hopefully this is something you can do in your spare time.
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- July 6, 2020 2771
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--------------------- Tuesday, July 7, 2020 -------------------------
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