Jacinta Bowler
Jacinta Bowler is a science journalist at Cosmos.
They were published in the Best Australian Science Writing 2023.
If you’ve ever wondered how size, mass, and relative densities of objects make up our Universe, a fun new study by Australian National University has some answers.
The paper, titled ‘All objects and some questions’ published in the American Journal of Physics features a mind melting graph featuring all objects in the Universe categorised into radius and mass.
A second graph shows temperature and density of the universe over time. The combined result, according to the researchers, is the most comprehensive chart ever created of all the objects in the Universe.
“When the Universe began 13.8 billion years ago in a hot Big Bang, there were no objects like protons, atoms, people, planets, stars or galaxies. Now the Universe is full of such objects,” says Associate Professor Charley Lineweaver.
“The relatively simple answer to where they came from is that, as the Universe cooled, all of these objects condensed out of a hot background.”
The graph showing radius and mass has sections where there’s nothing at all – a line of different sized black holes cuts off the objects in the universe from the section ‘forbidden by gravity’ while a smaller section towards the bottom is “forbidden” due to quantum uncertainty.
“Parts of this plot are ‘forbidden’ – where objects cannot be denser than black holes, or are so small, quantum mechanics blurs the very nature of what it really means to be a singular object,” says another of the researchers, Vihan Patel.
“At the smaller end, the place where quantum mechanics and general relativity meet is the smallest possible object—an instanton. This plot suggests the universe may have started as an instanton, which has a specific size and mass, rather than a singularity, which is a hypothetical point of infinite density and temperature,” Patel said.
Drawing the Universe in this way answers some questions but asks plenty more.
“These comprehensive pedagogical plots draw attention to the triangular regions forbidden by general relativity and quantum uncertainty and help navigate the relationship between gravity and quantum mechanics,” the researchers wrote in their paper.
“How can we interpret their intersection at the smallest possible objects: Planck-mass black holes (“instantons”)? Does their Planck density and Planck temperature make them good candidates for the initial conditions of the Universe?
“Our plot of all objects also seems to suggest that the Universe is a black hole.”
Luckily, although this works in theory, this is probably not the case in practice.
“On the larger end, the plot suggests that if there were nothing – a complete vacuum – beyond the observable universe, our Universe would be a large, low density black hole. This is a little scary, but we have good reason to believe that’s not the case,” Patel added.