What the Inflation Theory Can Tell Us about the Universe

From the Lecture Series: Great Heroes and Discoveries of Astronomy

By Emily LevesqueUniversity of Washington

The Inflation theory explained a number of observations, and agreed with the Big Bang model for how the universe began. Work by physicists refined this theory, imagining different possible trigger mechanisms that could kick off this exponential inflation and describing how it would stop, with the universe settling back to a slower rate of expansion and beginning to form the first elementary particles and atoms.

Concept of cosmic inflation
The best way to test the inflation theory is to make predictions. (Image: Maria Starovoytova/Shutterstock)

Is Inflation Theory a Valid Explanation?

Inflation is a popular and widely accepted explanation for the earliest moments of the universe, but there’s a catch. A theory that agrees with existing observations is very robust and satisfying, but successful theories should also accurately predict things. Put another way, theories can always be modified, or fine-tuned, by scientists to explain what we already see, but predicting something and then seeing it come to fruition is a much more powerful test of whether a theory is correct.

Physicists have argued about whether inflation has truly passed this test. Observations of the cosmic microwave background in the decades since Guth’s 1980 publication continue to agree excellently with the theory; however, the theory was designed to explain the cosmic microwave background. 

Explanations for the trigger mechanism of inflation and exactly how that exponential growth of the universe was driven, also remain elusive. Still, there are some possible new tests that could prove or disprove inflation theory.

One such test is searching for the gravitational signature of inflation. That early immense expansion of the universe should produce something called gravitational waves, ripples in the very fabric of spacetime, and the remnants of those ripples should still be detectable today, affecting the appearance of the cosmic microwave background.

A Dusty Universe

A nebula in space
Interstellar dust can get in the way of scientists having detailed observations of proof for inflation theory. (Image: Marcel Drechsler/Shutterstock)

In 2014, a team of radio astronomers operating a telescope at the South Pole held a triumphant press conference to announce that they’d found this tell-tale signature of inflation, seeing exactly what they’d hoped to see in detailed observations of the cosmic microwave background. However, a more careful analysis quickly revealed that their observations weren’t echoes of the universe’s earliest moments, but the result of light emitted by interstellar dust in our own Milky Way.

While it might sound silly to mistake light from bits of dust for the earliest echoes of the universe, dust is actually a ubiquitous and very challenging aspect of observational astronomy. Tiny dust grains floating through interstellar space can block or deflect light and even glow and emit light themselves, and they’re everywhere. Properly accounting for dust is a challenge that affects almost every subfield of astronomy. 

Seeing a signal from dust doesn’t mean that an underlying signal from inflation isn’t there; it just means we can’t be sure it’s there. As we improve our observing power, we may one day be able to take sensitive enough measurements to search for this signature of inflation and test, once and for all, whether Alan Guth’s theory is correct.

This article comes directly from content in the video series Great Heroes and Discoveries of AstronomyWatch it now, on Wondrium.

How It Started and How It’ll End

Inflation gives us a theory for how the universe began, and observations of the cosmic microwave background give us ways to test that theory, but how do we use observations to study how the universe will end? To explain this, we have to go back to Edwin Hubble. Hubble’s law was the equation showing us that objects that are farther away from us are moving away from us at greater speeds. 

Hubble and his contemporaries, including Georges Lemaître, recognized this fact as the sign of an expanding universe. Measuring H-naught, the Hubble constant—a number telling us how fast the universe is expanding—is a crucial puzzle piece in the math of how the universe is expanding.

A supernova
Studying supernovae is a popular way of measuring the Hubble constant. (Image: Martin Capek/Shutterstock)

There are many observational tools that astronomers can use for measuring the Hubble constant. One popular method uses a special type of supernova produced when a white dwarf—the leftover dead core of a star like our sun—explodes. Astronomers knew how bright explosions like this should be, which made white dwarf supernovae excellent standard candles.

Race to the Stars

By comparing a supernova’s apparent brightness to its expected brightness (and carefully accounting for complicating factors like dust!), observers could measure how far away that supernova was. These supernovae happen all over our universe, sometimes in very distant galaxies, and by the early 1990s two different research teams were racing to find ever-more-distant supernovae, so that they could measure their distances and speeds. 

By comparing the supernova’s distance to how fast it was moving away from us, astronomers could then very carefully test Hubble’s law and see whether this observed expansion was staying constant, slowing down, or speeding up.

One research team was created by Saul Perlmutter at Lawrence Berkeley National Laboratory; the other was founded by Brian Schmidt and Nicholas Suntzeff. Together the teams included dozens of scientists from all over the world. Working separately for many years, the two teams both reached the same surprising conclusion early in 1998: the fastest-moving supernovae were even further away than they’d expected. The universe’s expansion was accelerating.

Common Questions about What the Inflation Theory Can Tell Us about the Universe

Q: What are some of the problems with inflation theory?

Inflation theory has successfully managed to agree with existing observations of the universe. But an important part of a theory is to predict the future, and scientists still don’t agree if inflation theory has managed to do this. The theory helped to explain the cosmic microwave background, but it doesn’t tell us about the trigger mechanism of inflation and how the universe’s expansion was driven.

Q: Why is interstellar dust a challenging aspect of astronomy?

Bits of dust that remain from the earliest echoes of the universe have the ability to reflect, redirect, and even emit light which poses a challenge to astronomers trying to detect signals, such as those trying to observe for proof of inflation theory.

Q: How did the research teams that studied supernovae come to the conclusion that the universe’s expansion was accelerating?

After the inflation theory was coined, it was time to see if the universe was still expanding or not. Research teams managed to compare supernovae’s brightness to their expected brightness by calculating the influence of interstellar dust on measurements to measure how far away a supernova was. By comparing the supernova’s distance to the speed at which it moved away from us, scientists concluded that the universe’s expansion was accelerating.

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