The Universe has physical constants, such as the force of gravity that define everything. If these constants were any different, our Universe would look quite different. When you consider the types of objects that exist in our Universe – from quarks and bacteria to fleas and superclusters — different forces dominate their existence.
A fascinating new graph plots everything in the known Universe and shows us what’s possible. It also shows what types of objects are prohibited by the laws of physics as we understand them.
The graph, produced by astrophysicists Charles Lineweaver and Vihan Patel from the Australian National University’s Planetary Science Institute (PSI) is primarily a thought experiment that – the authors hope – will get people to think about all the unanswered questions we have about the Universe.
The graph and their paper, titled “All objects and some questions,” provides an overview of the thermal history of the Universe and the sequence of objects (e.g., protons, planets, and galaxies) that condensed out of the background as the Universe expanded and cooled.
Lineweaver told Universe Today the inspiration for producing this graph came from watching steam coming out of a kettle and seeing the hot water vapor condenses into droplets.
“This seemed like a general process that has happened multiple times as the hot dense universe cooled down as it expanded and condensed into various objects,” he said via email. “For example, as the hot dense plasma of quarks and gluons cooled, it condensed into protons and neutrons. And as the hot dense plasma of protons and electrons cooled down it condensed into atoms, known as recombination.”
This general process of “condensation,” Lineweaver said, seems to be underappreciated as a simple way to understand what happened as the universe cooled: the hot dense big bang condensed into objects.
Let’s take a closer look at Lineweaver and Patel’s graph:
The middle strip in the middle section marked “BBN” or Big Bang Nucleosynthesis has atoms and elements, with the atomic densities of things like bacteria, fleas, humans, whales, the Earth, Sun and stars.
Lineweaver says that astronomers (and astronomy enthusiasts) will enjoy the little rectangle in the middle of the top area. Here’s a zoom in view:
In it, we see main sequence stars, which when they run out of fuel, become white dwarfs, which eventually collapse into neutron stars, which eventually collapse into black holes. On this plot, black holes exist on the dark black line.
Returning to the main graph to see the larger view again, we see that the Hubble radius — which is the entire observable Universe — in on that line.
Does that mean the whole universe is a black hole? This graph seems to imply this might be true!
What does Lineweaver think about that possibility, that it’s plausible that what we see from inside our Universe is simply the result of being inside a black hole that formed from some parent Universe?
“This is an intriguingly weird idea,” he mused. “I often stay awake at night trying to bend my head around what it means… and then I fall asleep … inside a black hole. Based on the emails I have received I am not alone in suffering from a general relativistic insomnia.”
Lineweaver pointed out that contrary to common knowledge, black holes are not the densest things in the universe.
“The bigger the black hole, the less dense it is,” he explained. “That is why the whole universe could be a huge low-density black hole. Another interesting fact is when you trace the evolution of the whole universe back along this black hole line — all the way back to the beginning of the universe — the plot suggests that the initial condition of the universe was an instanton — the smallest possible black hole — an object that instantaneously evaporates (through Hawking radiation) and explodes at the highest possible temperature (the Planck temperature: 10^32 K.”
Whoa. Mind blown.
The area of the graph that might be most intriguing are two triangular regions that are ‘forbidden.’ Lineweaver and Patel explain this is 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.”
The boundaries of the plots and what lies beyond them are also a major mystery, as “the triangular regions forbidden by general relativity and quantum uncertainty and help navigate the relationship between gravity and quantum mechanics,” the scientists write.
“Our plot is an explicit and highly conventional extrapolation into very speculative territory,” Lineweaver explained. “If you repeat like a mantra the phrase ‘a proton is an ice cube’ that pretty much gets the basic idea across. This paper should help both students and experts articulate some very profound questions that we don’t know the answers to.”