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Until just a century ago, our galaxy was considered the only star family to occupy the Cosmos. Philosophers, notably Immanuel Kant in the 18th century, postulated the existence of other families of stars beyond our own. Unfortunately, their postulations – although correct – were not based on empirical data and therefore could not be proven.
This began to change during the 1920s and 1930s, as astronomer Edwin Hubble looked to other galaxies, using the 2.5-meter (100-inch) telescope recently built on Mount Wilson in the south. from California. For the first time, Hubble was able to clearly see the individual stars in M31 – the Andromeda Galaxy. For the first time, star families existed beyond the Milky Way.
Hubble also discovered something else: almost all galaxies are moving away from each other at tremendous speeds. He also discovered that these star clusters travel at a rate depending on their distance from us – more distant galaxies are moving away from us faster than local bodies. (Incidentally, there is nothing special about our position in the Cosmos. This same effect would be seen from any location in the expanding Universe).
The big question – which still needs to be answered precisely today – is how fast do they travel?
The expansion has started – WAIT!
Since the Big Bang, the Universe has expanded.
Edwin Hubble set out to measure the speed at which galaxies are moving away from each other. He discovered that galaxies obey a relationship, now known as Hubble’s Law, showing a linear relationship between the distance to a galaxy and its rate of recession. This speed is simply the result of measuring the distance to a galaxy and multiplying it by the Hubble constant.
The value of the Hubble constant is usually given in odd units, which may seem unknown – kilometers per second per megaparsec (km / s / Mpc). Let’s start at the end – a parsec is a unit of distance roughly equal to 3.26 light years. Therefore, one megaparsec (one million parsecs) is a distance equal to about 3.26 million light years.
If the Universe’s rate of expansion were 70 km / sec / Mpc, then a galaxy 10 megaparsecs away from us would – theoretically – be racing at (70 times 10, or) 700 kilometers per second. (It’s actually so close that the local gravitational effects would be significant, but this example shows the math). A galaxy twice this distance would have a recession rate twice as fast, and so on.
Then the question becomes – what is the value of the Hubble constant? It is one of the most important questions in cosmology and astrophysics today.
Ask the right question
Astronomers use several methods to measure the Hubble constant. However – a riddle has arisen. Observations from our local Universe produce different values from those obtained from studying the old (more distant) cosmos.
Typically, observations of galaxies in our galactic neighborhood show a Hubble constant of about 73 km / s / Mpc. Observations of ripples in space-time since the beginning of the Universe show a value of about 67 km / s / Mpc – a difference of almost 10%.
Knowing the true value of the Hubble constant would allow astrophysicists to determine a lot about the Universe, including its age, with a precision unknown today.
“The age of the universe is calculated using the rate of expansion from precise distance measurements, and the calculated age is refined based on whether the universe appears to be speeding up or slowing down, given the amount of matter observed in space. A rapid rate of expansion indicates that the universe did not need as long to reach its current size, and therefore it is younger than if it were expanding more slowly, ”says NASA.
A new study examines the average luminosity of stars in elliptical galaxies to more accurately measure their distances from Earth. Using this Surface Luminosity Fluctuation (SBF) technique, astronomers examined 63 giant elliptical galaxies near us, looking for an independent measure of the Hubble constant (often abbreviated as H-nothing).
The figure they found for H-nothing – 73.3 km / s / Mpc – is in agreement with three other methods used to measure the Hubble constant from nearby galaxies. These values reach on average 73.5 km / s / Mpc.
The farthest object in this study is seen 99 Mpc (about 323 million light years) from Earth – a small fraction of the size of the Cosmos.
“To measure the distances of galaxies up to 100 megaparsecs, it’s a fantastic method. This is the first article to bring together a large homogeneous set of data, on 63 galaxies, with the aim of studying H-nothing using the SBF method, ”explains cosmologist Chung-Pei Ma of the University of California at Berkeley .
Dark matter is a dish that is best served cold …
Elliptical galaxies are ancient families of older, predominantly red giant stars that provide a stable infrared signal across their extent. The images obtained by the Hubble Space Telescope were examined, measuring the brightness of each pixel in the image, comparing it to the average brightness seen in the image. Smoother images were seen of more distant galaxies, allowing astronomers to accurately measure the distance to these 63 targets.
Hit the play button above to watch an interview with Scott Lambros, head of instrument systems for the James Webb Space Telescope, talking about this revolutionary instrument that could help us understand one of the great mysteries of science .
The similarities between this measurement and other findings when examining local galaxies provide further evidence that the Hubble constant is likely to be around 73 km / s / Mpc.
So what about the lower values of H-nothing obtained from observations of the early Universe? If these numbers are incorrect, it would radically change the cold dark matter lambda (CDM) model of the Cosmos. This theory describes a lot the evolution of the Universe using only a few parameters.
“Once you can accept the universe as matter developing into nothing that is something, wearing stripes with checks becomes easy.” – Albert Einstein
Finding the CDM model incorrect would radically change our understanding of Cosmos. There is still a possibility that some currently unknown physics could reconcile the two radically different values of the Hubble constant. But this question remains one of the great mysteries of modern science.
The James Webb Space Telescope, scheduled to launch on Halloween 2021, will be 100 times more powerful than the Hubble Space Telescope. This revolutionary instrument will provide astronomers with their best measurements to date of the Hubble constant as measured by nearby galaxies. And, potentially, it could answer one of the greatest mysteries in the Cosmos.
Details of the study were reported in The Astrophysical Journal.
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