The Big Bang Theory is the most common way to think about how the universe began. It says that the universe as we know it began as an infinitely hot and dense single point that grew and spread over the next 13.8 billion years to become the still-growing universe that we see today.
Many things we know about the Big Bang are based on mathematical formulas and models because we don’t have the technology to look back in time and see how the universe came into being. But astronomers can still see the “echoes” of the expansion through the cosmic microwave background.
Although most astronomers agree with the theory, some theorists believe that there are other ways to explain the universe’s beginning, such as eternal inflation or a universe that moves around.
The classic Big Bang theory
For most human history, people who looked at the sky thought it was always the same and never changed. In the 1920s, Edwin Hubble took a look at this story and found that both galaxies outside the Milky Way were real and that their light looked stretched, which meant that they were speeding away from Earth.
Einstein’s field equations for general relativity recently made it possible to expand the universe. George Lemaître, a modern Belgian physicist, saw this as evidence that the universe was expanding. If you think back, you can see that the galaxies we see now must have started together in what Lemaître calls “the primordial atom.”
Fred Hoyle, an English astronomer, was the first to use the term “Lemaître’s idea.” It was March 28, 1949, when Hoyle came up with the phrase while defending his theory that the universe is eternal and that it makes matter to stop the universe from expanding. According to Hoyle, that the whole universe was made in a single big bang a very long time ago was wrong. In later interviews, Hoyle said that he didn’t mean to come up with a slur. The name stuck, which made some people angry.
Big Bang: Paul Steinhardt, a cosmologist at Princeton, said that the term is wrong. I think the Big Stretch would be a good fit. Imagining an explosion makes Steinhardt very confused, according to what he thinks about it. I believe there is a central point, more land, and a scene where light shrapnel moves faster than heavier pieces of debris. An expanding universe doesn’t look like that, he said. Large and small galaxies slide apart in the same way. There is no center or edge, and there is no difference in how big, and small they are (although more distant galaxies move away faster under the cosmologically recent influence of dark energy).
It doesn’t matter its name because the Big Bang theory has been widely accepted. After all, it can explain everything we see. The balance of light and particles like protons and neutrons during the first three minutes, for example, let early elements like helium and other light atoms form at a rate that predicted how many helium and other light atoms there were at the time.
People at New York University say a small window of time where nuclei could be made. When they couldn’t find each other after that, the universe kept growing, and it was too hot. Before the window, it was too warm.
It took 378,000 years for electrons and protons to cool down enough to form neutral hydrogen atoms, and the fog began to lift. When this happens, a lot of light is released. This light, which has since turned into microwaves, is the earliest thing researchers can look at right away. Many scientists think cosmic microwave background (CMB) radiation is the best evidence for the Big Bang. It’s called CMB radiation.
An explosive update of Big Bang theory
Eventually, as cosmologists went back even further in time, the story fell apart. Using equations from general relativity, we can figure out an infinite amount of heat and density at the start of the universe. This is called a singularity. Also, a single-origin didn’t match the smooth, flat CMB. It didn’t make sense. Fluctuations in the speck’s temperature and density would have caused different parts of the sky to have different properties—the CMB’s temperature changes by just a few degrees. Even the curvature of space-time isn’t very steep at first. This would mean an almost perfect balance between matter and curvature at first, which isn’t very likely.
During the 1980s, Alan Guth came up with a new way to think about what happened in the first fraction of a second. He said that the universe grew exponentially faster in the beginning than it does now. One day this process came to a stop, and when it did, a dense and hot mess of particles (but not infinitely so) took its place. : “In my own mind, I think of that as the Big Bang, when the universe started getting hot.”
Now, there are a lot of competing models for the inflation theory, which is what it’s called. Even though we don’t know much about how quickly the universe grew, the theory has become popular because it can explain the CMB, which doesn’t have any features at all. Inflation kept the small fluctuations that led to the formation of galaxy clusters, but it flattened the big ones. This is a very sweet story, Steinhardt told the people who helped make the theory come together. : “It’s the one we tell our children.”
Recent research has added two new twists to the inflation theory’s cosmic storyline, making it more complicated. It’s possible that inflation would have stopped in some places (like our observable universe) but kept going in others. This would have created an array of separate territories with “every imaginable” set of cosmological properties. People who study physics don’t like this “multiverse” picture because it makes an infinite number of untestable claims.
On the experimental side, cosmologists think that inflation should have made galaxy-wide gravitational waves in the CMB, just like it made small changes in temperature and density. Today’s experiments should be able to pick up on these things, but they haven’t shown up (despite one false alarm in 2014).
More precise measurements of the CMB could kill or support the many inflation models that are still in place. They don’t think the universe is very smooth because it began as a uniform whole and doesn’t need to be explained at all, but other physicists do.
Some theorists have stopped looking for ways to make the universe flat by inflating it, and they’re now looking for other ways to make the universe flat instead. Some people are working on a “big bounce” model that pushes the clock back even further to an earlier period of contraction that smoothed space-time and set the stage for an explosive expansion. Steinhardt is one of them. He hopes that soon, new signs and problems like the lack of primordial gravitational waves will give cosmologists a new story to tell. “Are there any other things that can be seen?” Steinhardt said, “Ask me again in a few years, and I’ll try to give you the answer.”