The universe is a big place, and it’s getting bigger all the time.
Space itself is stretching out, expanding in all directions.
But exactly how fast it’s expanding is still hotly debated,
and that number is vital to understanding the scale, size, and age of our universe.
The speed at which the universe is growing is called the Hubble Constant,
named after American astronomer Edwin Hubble.
In 1925, Hubble was the first to observe that objects astronomers once thought were spiral nebulae
were actually entire galaxies outside our Milky Way, and that space was much bigger than we thought.
Most of those other galaxies were red-shifted,
meaning the light they emitted was tinted red because their wavelengths had been stretched out.
So astronomers also realized that they were heading away from us.
In 1929, Hubble published a paper asserting the farther away a galaxy is, the faster it’s speeding off.
He’s long been given credit for this revelation, though it must be said he wasn’t the only one to have the idea.
A Belgian priest and cosmologist named Georges Lemaître
had come to the same conclusion two years earlier, but at the time his work was overlooked.
Establishing that the universe was expanding was the easy part,
but pinning down the rate it’s doing so has proven trickier.
There are two approaches to solving the problem.
One way is by examining the cosmic microwave background radiation,
the afterglow still lingering from the big bang.
By looking at how the matter in this early snapshot of the universe was distributed
and matching it up with features we see today, like galaxies and galaxy clusters,
we can determine the rate the universe expanded.
The other approach is to look for yardsticks out in the cosmos that will help us determine
how far distant objects are.
If we know how far away something is and determine its speed by how red its light is shifted,
that’s another way to arrive at the Hubble constant, denoted as H0.
Now, it’s not like we can pull out a tape measurer that’s millions of light years long,
so instead we look for what are called standard candles, objects with known brightnesses.
One standard candle often used is a category of stars called cepheids.
Cepheids grow brighter and dimmer at regular intervals, and the longer the interval, the brighter the star.
When we spot that telltale cycle of a cepheid, we can figure out how bright the star actually is,
and then figure out its distance from us since the apparent brightness drops over distance.
We even recently used these stars to map out our whole galaxy.
Now, here’s where the debate arises: these two methods both produce significantly different numbers.
Scientists who studied an extremely detailed map of cosmic microwave background radiation
concluded that the universe is expanding at a rate of about 67 kilometers per second per megaparsec.
That is, for every megaparsec about 3.26 million light years away an object is,
it zooms away from us 67 kilometers per second faster.
But by using cepheids and white dwarf supernovae to calculate H0,
a team of astronomers called SH0ES—because scientists can’t resist a punny acronym—
came up with the figure 74 kilometers per second per megaparsec.
Another team with the puntastic name H0LiCOW used quasars viewed through gravitational lenses
to arrive at an H0 of roughly 73 kilometers per second per megaparsec.
Most recently, another group of astronomers decided to identify a new type of standard candle altogether.
They looked for red giants that were just about to hit the predictable point
when they would start fusing helium in their cores and dim.
Using the brightest red giants as mileposts,
the team arrived at an H0 just under 70 kilometers per second per megaparsec.
That number is almost in the middle of the results produced by past studies.
Whether it’s the right number will take more research and debate.
The good news is we’re bickering over a difference of only a few kilometers per second.
Hubble’s first predicted rate of expansion was over 500 kilometers per second per megaparsec.
We haven’t concluded what H0 is just yet, but we’re much closer than we were 90 years ago.
It may be nice to hear that Lemaître finally got the recognition he deserves.
In 2018, the International Astronomy Union voted to recognize
what was known as the Hubble Law, by a new name.
They now call the idea that space is expanding the Hubble-Lemaître Law.
While we’re staring out into space, we’re also learning how life may have started.
Check out my video on a complex carbon atom we just spotted for the first time here.
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