Some of us blindly accept things the way they are. We exist. It doesn’t matter why, we just do. But for others, such as astrophysicist Duncan Meacher, accepting things the way they are without knowing why will never be enough. Alongside a bunch of other incredibly smart people at Penn State University’s department of physics, Duncan’s work helps humans to understand the state of our universe, its origins and how the forces within it work. space science earth science universe science
The answers to the universe’s big questions are rarely simple, but attempting to wrap our collective noggins around them can be a life-affirming experience. Those same answers also remind us of our relatively tiny place within the mind-blowingly large cosmos we call home. Here’s what Duncan told us when we asked him 10 of the most difficult space questions we could think of. space science earth science universe science
10. How old is the universe?
The current best estimate for the age of the universe is 13.8 billion years (the age of the Earth is about 4.5 billion years). space science earth science universe science
We have arrived at this number through several observations over the past century, the first of which came from Edwin Hubble who was able to determine the apparent speed at which distant galaxies appeared to be moving away from us by measuring the ‘redshifting’ of the light that is given off by them. This is analogous to the effect that makes an ambulance’s siren change pitch as it is moving towards or away from you, with light (or sound) waves being compressed (blueshifted) as the source moves towards you or stretched (redshifted) as it moves away. space science earth science universe science
In fact, the space between the galaxies is expanding which produces a cosmological redshift. By measuring the speed of the apparent recessional velocity of the galaxies, Hubble found that the further a galaxy was away from us, the faster it was moving. From this he was able to determine that all the matter in the universe must have, at one time, been a lot closer together. This is because the universe used to be a lot smaller before expanding to the size that it is today. space science earth science universe science
More recent experiments have been able to put tighter constraints on the age of the universe by measuring the cosmic microwave background (CMB). These experiments measured the background temperature of the universe which is produced from the leftover radiation of the Big Bang. As the universe expanded it cooled down until it reached the temperature that we measure today of 2.7 Kelvin (or minus 270ºC). The observations of core collapse supernova, which are the explosive last dying moments of stars that shine brighter than whole galaxies, allow us to measure the distance to the galaxies they’re within. By combining the results of the nearest and most distant observations, we are able to arrive at the age of the universe. space science earth science universe science
09. Why is the universe expanding?
In the first few moments after the Big Bang, the universe was in an extremely hot dense state, which led to its expansion. This started with the initial inflation of the universe which provided the energy for its expansion. Since that initial inflationary period, the universe has continued to expand, with the rate of expansion increasing over time due to the effects of dark energy.
Exactly what this dark energy is is unknown, but it is thought to produce a negative pressure. This acts as a repulsive force that causes the universe to expand. There are currently many experiments underway that are trying to understand exactly what dark energy is, but a lot of work is still needed before we are able to reach an answer.
08. Why did the Big Bang take place?
The question of ‘why’ is impossible for us to answer, and leads into much deeper philosophical and metaphysical questions. All we are able to do is determine what happened from the moments after the Big Bang to the present day.
Right after the Big Bang, the universe was in a very, hot dense state. This meant that light rays were unable to travel far as they were constantly being absorbed and then re-emitted. It wasn’t until 380,000 years after the Big Bang that the universe had cooled enough to allow the light rays to travel without being interrupted.
One possible way that we may be able to investigate earlier times in the universe is to measure the gravitational waves that were emitted from the Big Bang and other cosmological processes. Gravitational waves are ripples in the fabric of space-time that are produced by the interactions of very massive objects and are able to carry a lot of information about the sources that produced them.
The analogous way to think of gravitational waves is that they are the sound that the universe makes, as opposed to light which we are able to see. However, gravitational waves interact very weakly with matter and so it is very difficult to detect them. The LIGO experiment in the US has just finished its first observing run with its upgraded detectors to make the first direct detection of gravitational waves from astrophysical events. These detectors won’t be sensitive enough to see any signals from the Big Bang, but it is hoped that some planned future detectors will be able to probe the first few moments of our universe.
07. How and when will the universe end?
There are many theories for how the universe could end. The two most commonly suggested ones are the Big Crunch and the Big Rip, which both result in the heat death of the universe. The difference between these two outcomes depends entirely upon the matter and energy content of the universe. The universe is currently expanding, and the rate at which it is expanding is also increasing. If the matter content of the universe is high enough then the gravitational attraction would be strong enough to slow down and stop this expansion, then cause everything to start being pulled back together. The Big Crunch is the point at which all matter is brought back to a single point—a reverse of the Big Bang.
However, if the matter and energy density isn’t large enough then the universe will keep on expanding forever, effectively ripping itself apart until it reaches a state of thermal equilibrium. All stars would run out of fuel and either explode or collapse to form compact objects such as white dwarfs, neutron stars or black holes, which will then radiate away all their heat.
Galaxies will continue to fly apart from each other, to the point where if you looked up at the night sky all you would see is darkness. Even black holes, whose gravitational pull is so strong that even light can’t escape from them, will evaporate during a process called Hawking radiation.
As for how long this will take, there is no definite answer but it is expected to be a very long time away. The time needed for all the stars to burn up and explode and for galaxies to be ripped apart should take trillions of years. As for the time needed for black holes to evaporate, some estimates put this at over 10^100 (10 with 100 zeros after it) years. So a very long time!
06. What is a parallel universe, and could there potentially be ones that contain doppelgängers of everyone on Earth?
There are several different meanings used to describe parallel universes. The first is based on the theory that the observable universe is part of a much larger universe, comprised of an infinite number of universes. Each universe is spatially separated enough within its own bubble so that someone in one universe would never be able to observe a different universe. In this model, it would be very unlikely that there would ever be another ‘you’ contained within a different given universe bubble. As this universe would be infinite, however, given an infinite amount of time, that probability goes to one. In fact, there would be an infinite number of you!
Another way that parallel universes are thought of is based on the so-called ‘many worlds’ interpretation. This is built on the premise that for every random choice made, the universe will split into two universes, with each one having a different outcome. This doesn’t apply to everyday choices such as the flipping of a coin to get either heads or tails, but at a quantum mechanical level.
Quantum mechanics works by assigning probabilities to outcome instead of stating exactly when or where something will happen. A common example is the Schrödinger’s cat thought experiment. This says that if a cat is placed within an isolated box with a sealed container of poison that will break when some random process occurs, then from an outsider’s perspective the cat is both dead and alive at the same time. It is only by opening the box to check on the cat that we are able to determine what has happened to it.
In the many worlds interpretation, if someone were to open the box and find the cat alive, then there would be a split universe where the cat is found dead. In this case, in both parallel universes there would be a copy of you, with the only difference being that one would have a live cat and the other would have a dead cat. However these parallel universes are intrinsically and fundamentally separate; we can never communicate between them.
05. If the universe is potentially infinite, can we safely say that other intelligent life exists somewhere within it?
In the 1960s, astronomer Frank Drake produced the Drake equation. This equation tries to determine a statistical expectation of how many other intelligent civilisations may exist in the universe. It does this by taking into account factors such the rate at which stars are formed in the universe, the fraction of these star systems that will form planets, the number of these planets that will be able to support life, and so on. Given the sheer scale of the universe and the amount time that it has been around, it’s not hard to imagine that there are, or have been, thousands or even millions of other intelligent civilisations out there.
The question that usually follows this is ‘Will we ever communicate with alien life forms?’. This is harder to answer and is covered by the Fermi paradox. It states that given the billions of stars in our galaxy and billions of galaxies in the universe, there should a large number of alien civilisations out there. So given that there should be a large number of alien civilisations out there, why haven’t we seen or heard anything from them?
This is a very difficult question to answer, and many answers have been put forward to try and explain why. One of the simplest possible reasons is that interstellar travel is very difficult. Travelling at close to the speed of light, it could still take a lifetime for someone to reach another habitable planet.
04. What is it like inside a black hole?
When massive stars reach the end of their lives, with all of their fuel having been consumed, they will undergo a huge explosion which we call a core collapse supernova. This causes most of the outer layers of the star to be expelled and will leave behind just the matter from the inner core which, if there is enough mass, will very quickly form a black hole.
For this to happen, the gravitational attraction from all the remaining matter needs to be larger than the repulsive force that holds it up. In the star, this repulsive force came from the radiation pressure produced from the nuclear burning (fusion) of the fuel in the core. In post-main sequence stars such as white dwarfs or neutron stars, this force comes from trying to push all the sub-atomic particles together, such as electrons and protons to form neutrons.
If there is enough mass to overcome these forces then gravity will win and everything is compressed into a space that is infinitely small, which is called a singularity. Outside the singularity there is a boundary which we call the event horizon. It is impossible for us to look at anything that lies inside within this boundary as past this point the gravitational pull is so strong that not even light is able to escape from it, hence why it’s black.
If you were to pass the event horizon, it would be a very strange world. If it was a relatively small black hole, you would very quickly be spaghettified. The gravitational pull on your feet would be much greater than that on you head, and your body would literally be turned into spaghetti! With more massive black holes, such as the one at the centre of our galaxy, the gravitational tidal forces are a lot less severe, and so you would be able to survive, though it is very difficult to say what you would see or experience inside.
03. Why does a person in space age at a different rate to someone on the Earth?
Time will pass at different rates for two observers, one located on Earth and the other located in space, due to the effects of general relativity. The closer someone is to a massive object, the slower time will pass for them, as measured by someone located far away. The more massive the object, the slower time will appear to flow.
In the most extreme case of someone falling into a black hole, an observer would see them moving slower and slower as they fall towards the event horizon, until they eventually appear to stop. However from the point of view of the person falling in, time would appear to flow normally.
In the case of the earth, this effect is extremely small, though must still be accounted for with technologies such as global positioning satellites (GPS). Without accounting for general relativity, GPS would very quickly become very inaccurate.
02. What happens when you travel faster than the speed of light?
With our current understanding of the laws of physics it is impossible for anything to travel faster than the speed of light. Anything with mass—be it a particle or a spacecraft—would need an infinite amount of energy to equal the speed of light.
However, there are ways around this which are regularly used in science fiction, such as warp drives or wormholes. The premise behind these is that the spaceship doesn’t actually travel faster than the speed of light. Instead, they shrink or fold space-time around them. For instance, a warp drive would compress the space in front of a ship so that when the ship travels through that space, contained within a warp bubble of ‘normal’ space-time, it would be able to cover great distances while never actually breaking the speed of light. Unfortunately this requires the use of negative mass which, for time being, remains a theoretical concept.
Wormholes work by folding space-time so that two distant points become connected, allowing a spaceship to travel instantaneously between them. While this is a highly theoretical concept, as with the warp drive, there is nothing in the laws of physics that would prevent this.
01. Is time travel possible, and if so, how could it hypothetically be achieved?
Yes, but not in the way you think. As already mentioned, if you are close to a very massive object, time for you will flow slower than for someone else far away. The same effect will happen for someone moving at speeds close to the speed of light.
If you left earth on a spaceship and went on a round trip to the nearest star at close to the speed of light, 10 years would pass for you while 100 years would have passed for everyone back on earth. The faster you were able to travel, the slower time would flow for you compared to everyone else.
Using this method it would be possible to travel into the future, but it would be a one way trip. The question of being able to travel backwards in time is a lot more complicated. It has been theorised that wormholes may provide a means of being able to travel backwards and forwards in time, but again this is all highly theoretical.
There are also some paradoxical questions that arise when considering travelling backwards in time. The best-known of these is called the grandfather paradox. It states that, if you were to travel backwards in time and do something that results in your grandfather being killed before he met your grandmother, then you would never have been born. But if you were never born, there would be no one to go back in time to kill your grandfather, and he would be able to meet you grandmother and you would be born, to go back in time and kill your grandfather, and so on.