The Lambda Cold Dark Matter (Lambda-CDM) model, which is the current accepted standard for how the universe started and developed, says that the normal matter we see every day only makes up about 5% of the universe’s density, while dark matter makes up 27% and dark energy, which is still a theory, makes up the other 68%.
But a new study questions whether dark energy even exists. It does this by pointing to computer simulations that show that the gap in the theory that dark energy was thought to fill disappears when the changing structure of the universe is taken into account.
Einstein’s general theory of relativity, which came out in 1915, is the basis for the accepted story of how the universe started. This story says that the Big Bang started the expansion of the universe about 13.8 billion years ago.
The problem is that the equations at work are very hard to understand, so physicists tend to simplify parts of them to make them easier to use. When models are built up from these simplified versions, small holes can grow into big differences.
“Einstein’s equations of general relativity that describe the expansion of the universe are so complex mathematically, that for a hundred years no solutions accounting for the effect of cosmic structures have been found, we know from very precise supernova observations that the universe is accelerating, but at the same time we rely on coarse approximations to Einstein’s equations which may introduce serious side effects, such as the need for dark energy, in the models designed to fit the observational data.” says Dr László Dobos, co-author of the new paper.
Dark energy has never been seen directly, so we can only learn about it by looking at how it affects other things. Its properties and existence are still just theories, so it can only be used to fill in gaps in existing models.
In the 1990s, Type Ia supernovae were used to suggest that the mysterious force was speeding up the expansion of the universe.
These bright spots are sometimes called “standard candles” because they always shine at the same peak brightness. Astronomers can figure out how far away an object is by measuring how bright the light is when it gets to Earth.
This research helped the idea that dark energy is speeding up the expansion of the universe catch on, and the scientists who worked on it won the 2011 Nobel Prize in Physics for their work. But other studies have questioned whether or not that conclusion is true, and some scientists are trying to get a more accurate picture of the universe by using software that can handle all of the complexities of the general theory of relativity better.
According to a new study from Eotvos Loránd University in Hungary and the University of Hawaii, the gap that dark energy was “invented” to fill might have come from parts of the theory that were left out to keep things simple. Based on how the universe is put together on a large scale, the researchers set up a computer simulation to show how the universe came to be. This structure seems to look like “foam.” Galaxies are found on the thin walls of each bubble, but most normal and dark matter is missing from the large spaces in the middle.
The team ran a computer simulation to see how gravity would affect matter in this structure. They found that the universe would not grow in a smooth, even way, but that different parts of it would grow at different speeds. Importantly, though, the average rate of growth is still in line with what has been seen and points to faster growth. The Avera model is what the team came up with in the end.
“The theory of general relativity is fundamental in understanding the way the universe evolves, we do not question its validity; we question the validity of the approximate solutions. Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion. These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy.” says Dobos.
If the research holds up to scrutiny, it could stop physicists from chasing the ghost of dark energy and change the way they study the universe.