What is space made of, how fast is it expanding? - study

What is the universe made of and how fast is it expanding? These are important questions and relate to the mystery of the Hubble constant. A new study sheds light, but questions still loom ahead.

 G299 was left over by a particular class of supernovas called Type Ia. (photo credit: NASA/CXC/U.Texas)
G299 was left over by a particular class of supernovas called Type Ia.
(photo credit: NASA/CXC/U.Texas)

What is the universe made of, and how fast is it expanding? These are questions that may have been answered by a new study, though it also reaffirms major questions about the mysteries of the cosmos that will be extremely challenging to answer.

The findings of this study, published in the peer-reviewed academic periodical The Astrophysical Journal, cements the Standard Model of Cosmology with more concrete evidence to back it up, thanks to having gotten a better picture of how the universe is made up.

According to the study, based on the Pantheon+ analysis, the universe is composed of roughly two-thirds dark energy and one-third matter, with most of the matter being dark matter. In addition, the universe is also rapidly expanding at a faster pace over the last few billion years. But now, cosmologists studying the inner workings of the universe are at a crossroads, and while Pantheon+'s data is answering some questions, it also makes things even more confusing.

The makeup of the universe

According to the Standard Model, the universe consists largely of three things: Dark energy, dark matter and baryonic matter, the latter of which is just ordinary regular matter

But this is a big mystery because no one actually fully knows or understands what dark energy or dark matter actually are.

 Dark matter and gas (Illustrative). (credit: Wikimedia Commons) Dark matter and gas (Illustrative). (credit: Wikimedia Commons)

Essentially, dark energy is the energy that keeps the universe expanding, as well as accelerating. 

But despite not knowing what dark energy is, scientists do know it exists, thanks to measuring supernovae, which itself helped form the theory that the universal expansion is accelerating.

Dark matter is just a kind of matter, and it makes up around 85% of the universe's matter. But while we do know that it is matter, we don't know anything else other than that.

The reason is that dark matter is, as the name implies, dark, which means it has essentially zero interaction with electromagnetic radiation or light whatsoever.

In other words, it's invisible. Dark matter is dark because we can't see it.

But we do know it exists thanks to gravity, though how scientists are able to detect where it is can be complicated and involves a technique called gravitational lensing.

Now, scientists have always thought that roughly speaking, the universe was mostly dark energy and dark matter, with regular matter making up just around 5% of the cosmos. And this study, Pantheon+, seemingly confirms it.

 Artist’s impression of two white dwarf stars destined to merge and create a Type Ia supernova. (credit: European Southern Observatory/Flickr) Artist’s impression of two white dwarf stars destined to merge and create a Type Ia supernova. (credit: European Southern Observatory/Flickr)

Pantheon+ analyzed a dataset of over 1,500 Type Ia supernovae. Regular supernovae are, simply put, when a star dies by becoming so dense that it explodes, blasting material all throughout space. A Type Ia supernova, however, is a bit more specific. This is what happens when a white dwarf star, which itself is the remnant of a dead medium-sized star like the Sun, orbits either a regular star or another white dwarf in what is known as a binary system, with the white dwarf essentially absorbing the other star in a process known as accretion until it passes the limit and triggers a thermonuclear reaction. 

These blasts are incredibly bright and can be seen from tens of billions of light-years away. And considering how long ago that would be, scientists are able to calculate the brightness of supernovae to measure age and distance. This is essential in measuring the distance in the universe and the rate of expansion, which is why these supernovae have often been dubbed "cosmic milepost markers."

Not only that, but there is another factor to consider, cosmic microwave background, the remnant of the Big Bang. This also help measure the rate of expansion.

Pantheon+ itself is a follow-up to an earlier analysis called Pantheon, and includes around 50% more supernovae data in its dataset. After compiling it all, the researchers behind Pantheon+ combined it with measures like cosmic microwave background, as well as the data from SH0ES (Supernova H0 for the Equation of State)

The latter is a collaborative scientific measure that worked on studying the rate of universal expansion. This, itself, means solving one of the greatest cosmic mysteries of them all: 

What is the Hubble constant? 

The Hubble constant itself is just the rate of expansion of the universe. 

This idea was first discovered around a century ago by astronomer Edwin Hubble, who found other galaxies outside the Milky Way and found that they were constantly moving away.

Not only that, but the farther an object is from the Milky Way, the faster it seems to be moving.

Now, we have no idea how fast everything is moving, but understanding this could help us understand the rate of expansion, the consistent speed at which the universe expands in something that has become known as the Hubble constant. Knowing that can help tell us when it began.

To put it more simply, it could possibly be used to essentially tell us just how old the universe is.

But some recent research had also implied that the Hubble constant may not exactly be constant, since the rate of acceleration doesn't match up to predictions.

The early prediction of the Hubble constant that was for a long time widely accepted was 67.5. 

However, SH0ES and Pantheon+ together have managed to calculate it as 73.4, and they said there was just a one-in-a-million chance they were wrong.

How fast is space expanding?

To extremely simplify a complex cosmic measurement, space expands at a rate of over 264,000 kilometers per hour for every megaparsec, which is a space of 3.26 million light years.

But measuring with cosmic microwave background rather than supernovae always says that the Hubble constant is much slower. 

And herein lies the problem that has become known as the Hubble tension, a massive confusion in the field of astrophysics. And rather than getting us closer to solving it, it just reaffirms how confusing of a divide this is.

"We thought it would be possible to find clues to a novel solution to these problems in our dataset, but instead we’re finding that our data rules out many of these options and that the profound discrepancies remain as stubborn as ever," lead author Dillion Brout explained.

"We thought it would be possible to find clues to a novel solution to these problems in our dataset, but instead we’re finding that our data rules out many of these options and that the profound discrepancies remain as stubborn as ever."

Dillion Brout

So what could possibly explain this?

Well, as Brout suggests, it might be just parts of physics from the early years of the universe that we don't yet understand. However, such a theory is very much unverified and lacks evidential ground to stand on. 

Solving this mystery will continue to be a massive challenge for scientists in the years to come.

 The universe is filled with mysteries that scientists struggle to answer (Illustrative). (credit: PIXABAY) The universe is filled with mysteries that scientists struggle to answer (Illustrative). (credit: PIXABAY)

Why do we care about the expansion of the universe?

This is a complicated question and the answers are even more complicated, but to oversimplify it, it could tell us, or give us clues about, the ultimate fate of the universe.

A prominent theory is that the universe will keep expanding, matter will become less dense as a result and soon all matter will just disintegrate in what is called the heat death of the universe.

Other theories exist too, such as the Big Crunch or Big Rip, but ultimately we have no idea what will happen.

To many, this is a scary thought, but to others, it is simply more challenges and avenues for science to explore.