Astronomers and astrophysicists need to know how fast the Universe is expanding to determine the past — and future — of the Cosmos. However, different methods of measuring this expansion rate produce different values — why does this happen, and what does that mean for the Universe in which we all live?
When astronomer Edwin Hubble first discovered the Universe is expanding, he also found that galaxies further away from us are traveling at a faster rate than galaxies closer to us. This relationship became known as the Hubble constant.
Since 1929, when the Hubble constant was first proposed, astrophysicists have made significant advances in their understanding of the expanding Universe. The Hubble constant (H0) is usually referred to in units of kilometers per second per megaparsec (km/s/Mpc), where a megaparsec is a unit of distance equal to roughly 3.25 million light years.Hubble originally believed galaxies were expanding at a rate of 500 km/s/Mpc.
“The expansion age of the Universe inferred from this was only [two billion years], but by the 1930’s, radioactive dating of rocks had already shown geologists that the age of the Earth was [three billion years],” John Huchra of Harvard University reported in 2008.
This value calculated by Hubble turned out to be much too high, and currently accepted figures tend to hover around 70 km/s/Mpc — a galaxy sitting 3.25 million light years from us expands away from us at around 70 km/s (157,000 MPH). A galaxy twice as far from us would expand away from us at twice that speed. This value appears to change over time, but is still referred to as a constant, particularly when measuring galaxies not far from our own.
The fact that galaxies appear to expand at a rate dependent on their distance from the Milky Way does not mean that we occupy any special place in the Universe. Observers in any galaxy would also see other groups of stars expand away from them in an identical fashion, similar to raisins in bread as the loaf expands in an oven.
Here be Dragons
Astrophysicists have a number of options open to them when they measure the Hubble constant. However, each method of doing so produces different results. Although these values “only” range between around 68 and 78 km/s/Mpc, this is enough of a difference to result in radical changes to our understanding of the nature of the Cosmos.
One of the central scientific goals of launching the Hubble Space Telescope (HST) into orbit was to measure the expansion rate of the Universe. The name of the world’s best-known space telescope comes from the early-20th-century astronomer who first noted how other galaxies appear to be racing away from our home family of stars.
By knowing the expansion rate of the Cosmos, it is possible to calculate its age. However, various values of the Hubble constant between 68 and 78 km/s/Mpc result in a one billion year difference in the age of the Cosmos.
An accurate value of the Hubble constant is also needed to determine the shape of the Universe, and to determine if the Cosmos will expand forever, or collapse in the distant future, potentially resetting the Universe for another Big Bang.
During the summer of 2019, more than 100 researchers met at the Kavli Institute for Theoretical Physics at the University of California to discuss the varying measurements of the Hubble constant, and what it could mean for the Universe.
Each method of measuring the Hubble constant can be divided into one of two classes — those based on measurements of stars close to us (in the late Universe), and from light produced in the early Universe.
The first reliable method of measuring the distances to galaxies, developed by astronomer Henrietta Swan Leavitt, relies on measuring the apparent brightness of Cepheid variable stars. These bodies shine at a predictable levels, and by measuring how bright they appear from Earth, it is possible to calculate how distant the star (and its accompanying galaxy) is from our home world. Examining Cepheid stars, researchers measure the Hubble constant to be approximately 73.9 km/s/Mpc.
Using data from the Planck space telescope (and other instruments) to study the oldest light in the Universe, researchers developed the Lambda-CDM model. This theory postulates the Universe is composed of roughly five percent ordinary matter, 27 percent dark matter, and 68 percent dark energy (which powers the ever-quickening expansion of the Universe). This Lambda-CDM model suggests a value for the Hubble constant of around 67.4 km/s/Mpc, much lower than the value obtained from observations of Cepheid variable stars.
Astronomers can also measure the distances to distant galaxies (and the Hubble constant) is by studying Type 1a supernovae, stellar explosions that erupt at precise luminosities, at least in the modern Universe.
“There are very different and independent ways (with totally different instruments and scientific tools) to measure the H0 on the basis of the early Universe, and the same goes for the late Universe. What is interesting is that all the measurements of one type are in mutual agreement with one another, at an exquisite precision of 1 or 2%, as are those of the other type, with the same great precision; but when we compare the measurements of one class with those of the other, the discrepancy arises,” Licia Verde of the University of Barcelona explains.
Although a roughly seven percent difference in the measured values of H0 may not seem significant, higher or lower values would foretell far different life stories for the Cosmos.
If the Hubble constant were around 67 km/s/Mpc, then our Universe would be approximately 13.8 billion years old. If H0 were actually 74 km/s/Mpc, then just 12.8 billion years would have elapsed since the Big Bang.
“The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades. This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur just by chance,” explained Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University.
Actually, it DOESN’T Depend on Who You Ask…
Anomalies in the figures do not depend on the instrument, nor the investigator carrying out the study — each method of measuring the value returns similar values to other studies carried out using the same method. However, each method produces its own characteristic value for H0.
One way of explaining this conundrum is that the Lambda-CDM model of the Universe may be in error, but this theory has been extremely successful in making predictions about the Universe, a characteristic of highly-accurate scientific models.
“If we know the initial amounts of stuff in the universe, such as dark energy and dark matter, and we have the physics correct, then you can go from a measurement at the time shortly after the big bang and use that understanding to predict how fast the universe should be expanding today. However, if this discrepancy holds up, it appears we may not have the right understanding, and it changes how big the Hubble constant should be today,” Riess stated.
“Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy.” — Albert Einstein
However, it may still be possible that some tweaking of the Lambda-CDM model could account for the variations seen by astronomers in the values they obtained for the Hubble constant. A sudden expansion of the early Universe, fueled by early dark energy, could account for the discrepancies seen in the data.
Some astrophysicists have also suggested a class of subatomic particles traveling at nearly the speed of light could account for the discrepancy in values of H0. It is also possible that dark matter (which holds galaxies together) could interact with ordinary matter more strongly than once believed, resulting in the differing figures seen in the data. However,none of these ideas have yet been shown to be correct.
Until we understand why values found for H0 are so dependent on the method by which it is determined, we won’t know the age of the Universe, how long it will exist, or its ultimate fate.
Analysis of the study was published in the journal Nature Astronomy.
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