Gravity’s Role in the Story of Our Universe

Dr Katy Clough

by Institute of Physics: Oxford Centre


Einstein’s strange and beautiful theory of general relativity has provided many a plot line in science fiction with its time-warping effects around black holes. But gravity also plays a major role in the story of our universe as a whole, not least in the question of how the universe “began”. Gravity tells us how to rewind the universe to its beginning, but when we do, we seem to find more questions than we answer. Can gravity really tell us where we came from, and might this give us a clue as to where we are going?

In this Dr Clough started by introducing us to Einstein’s theory of general relativity, and explained why it is fundamental to understanding how our universe changes over time. She then discussed the problems we find when we naively follow this theory to its logical conclusion (or rather, since we travel back in time, to its logical beginning). This led us to our goal – a discussion of recent developments in using computational simulations of gravity to inform our search for answers in early universe cosmology.

Spoiler alert! This is a story with no satisfactory ending – yet! But she hoped, nevertheless, to provide a compelling narrative on the ways in which scientists are attempting to push back the boundaries of what is known using gravity.


image credit: NASA/Planck collaboration

About the speaker

Dr Clough is a Postdoctoral Research Assistant in Theoretical Cosmology and Gravitational Physics in the Beecroft Institute for Particle Astrophysics and Cosmology at University of Oxford. Her work involves computational simulations of strong gravity environments in the early Universe and around black holes, as a way of understanding the origins of the universe and the nature of dark matter.

The lecture

The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope Dr Clough and my readers will forgive any mistakes and let me know what I got wrong.


Diagram of evolution of the (observable part) of the universe from the Big Bang (left), the CMB-reference afterglow, to the present. NASA/WMAP Science Team – Original version: NASA; modified by Cherkash

Timeline of the universe. A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe.

We have a very good idea of the sequence of events from when the first stars emerged to our current time, but that big white light at the beginning of the Universe is very annoying. The situation can be summed up in the famous cartoon below.


Perhaps the label “quantum fluctuations” should be replaced with “We have no idea”.

The aim is to try and understand what this white light is.

Except for a relatively small number of people we know the Earth is spherical but what shape is the Universe?

The answer is that the Universe is spherical too.

In order to start understanding this we can use the Earth as a model.

One of the problems we have is that maps are drawn on flat surfaces. If the Earth were flat, like a map, we would expect the following to happen if two people, 2000km apart, decided to walk North from the Equator. They would travel in a straight line, keeping a constant distance between them.


They should find that they are still the same distance apart when they reach Europe because parallel lines never meet on a flat surface.

However, we know the Earth is a sphere and travelling in a straight line in the same direction on the surface will cause the distance between the walkers to decrease and they will meet at the North Pole.


The above image shows what happens when two people walk North in straight lines on Earth.


It’s easy to understand why our ancestors thought the Earth was flat because they didn’t travel very far from their homes and over small distances the curved surface of the Earth does appear flat. The above image shows that initially the walkers would be walking in straight lines, keeping roughly the same distance apart whether the Earth was round or flat. The Earth’s curvature only becomes apparent if the walkers travel a long distance.

But we have known the Earth was round for a very long time.

The earliest documented mention of the spherical Earth concept dates from around the 5th century BC, when it was mentioned by ancient Greek philosophers.


Flat Earth view versus curved Earth view. If the two people were scientists, they would know the Earth is round so they will expect to meet each other at the North Pole, but if they weren’t scientists and believed that the Earth was flat they would think that there was a mysterious attractive force bringing them together at the North Pole.

The above right diagram is a good analogy for gravity, a mysterious attractive force. However, it could be argued that gravity is not actually a force but a consequence of the fact that we are moving on a curved spacetime in a curved Universe exactly like the two scientists moving on a curved surface. It’s this curving of the Universe that makes it look like things are being attracted towards each other, just like the two scientists moving on the curved surface of the Earth.

We can’t all fly into space and look at the Earth but we know from the Apollo missions, satellite imaging and pictures taken from the ISS that it is round.

The Apollo program, also known as Project Apollo, was the third United States human spaceflight program carried out by the National Aeronautics and Space Administration (NASA), which succeeded in landing the first humans on the Moon from 1969 to 1972.


Oh look, an image taken by Apollo 8 crewmember Bill Anders on December 24, 1968, at mission time 075:49:07 [8] (16:40 UTC), while in orbit around the Moon, showing the Earth rising for the third time above the lunar horizon, and its round.


The International Space Station (ISS) is a modular space station (habitable artificial satellite) in low Earth orbit. It is a multinational collaborative project between five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada).

Video of Earth Seen from The ISS Live and it’s round.

However, we can’t imagine a curved Universe because we can’t move outside it but we can do the same sort of trick as before but replacing the two scientists with two astronauts floating about in space and replacing the space axis with time.

If the Universe were flat then the astronauts, providing they were not near any stars or planets or each other, would be the same distance apart after a certain time as they were at the start. They don’t move apart.

It’s important to remember that time can only move in one direction, forwards. On Earth we can move in any direction but with space-time we can only move forwards (however I would quite like to be 30 again).

If the Universe is curved then we would expect the astronauts to move apart. The distance between them would increase.

We can’t actually stick two astronauts in outer space and leave them there. For one it would be completely immoral but secondly, they would be too far away to monitor.


If the Universe were flat then astronauts who start out not moving relative to one another stay this way. If the Universe were curved then astronauts who start out not moving relative to one another move apart.

Luckily, we do observe this moving apart with galaxies.


Our Universe is expanding and this expansion is accelerating. Galaxies are moving apart at an increasing rate.

Hubble’s law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. (below left)


Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. (above right)

Georges Henri Joseph Édouard Lemaître, RAS Associate (17 July 1894 – 20 June 1966) was a Belgian Catholic priest, mathematician, astronomer, and professor of physics at the Catholic University of Louvain. He was the first to identify that the recession of nearby galaxies can be explained by a theory of an expanding universe, which was observationally confirmed soon afterwards by Edwin Hubble.

If we have two galaxies 1 megaparsec apart we find that after one second this distance has increased to 1Mpc + 70km.

The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System. One parsec is approximately equal to 31 trillion kilometres and equates to about 3.3 light-years ( a light year is the distance travelled by light in a year).

The 70km is due to the fact that the Hubble constant has a particular value now but if we were to check in a few million years it could be completely different.

The Hubble constant (named after Edwin Hubble) is a unit that describes how fast the universe is expanding at different distances from a particular point in space. It is one of the keystones in our understanding of the universe’s evolution — and researchers are constantly debating what its true value should be and whether it is constant at all.

The fact that the Universe is expanding at a faster rate tells us that spacetime is becoming more curved.

The beginning of the Universe

We are told that the Universe began with a Big Bang but this is too simple a view.


It should be noted that the term “Big Bang” was actually a derogatory comment by Fred Hoyle. He found the idea that the universe had a beginning to be pseudoscience, resembling arguments for a creator, “for it’s an irrational process, and can’t be described in scientific terms” (see Kalam cosmological argument). Instead in 1948 he, along with others, began to argue for the universe as being in a “steady state” and formulated the Steady State theory. The theory tried to explain how the universe could be eternal and essentially unchanging while still having the galaxies we observe moving away from each other. The theory hinged on the creation of matter between galaxies over time, so that even though galaxies get further apart, new ones that develop between them fill the space they leave. The resulting universe is in a “steady state” in the same manner that a flowing river is—the individual water molecules are moving away but the overall river remains the same.


Sir Fred Hoyle FRS (24 June 1915 – 20 August 2001) was an English astronomer who formulated the theory of stellar nucleosynthesis. He also held controversial stances on other scientific matters—in particular his rejection of the “Big Bang” theory, a term coined by him on BBC radio.

The simple view of the Big Bang is an explosion at the start of time that threw everything apart and is the reason why all the galaxies are moving away from each other in different directions because they have this left-over movement from the explosion.

This is not the image we should have in mind. The astronauts mentioned earlier hadn’t been in any explosion. They were initially just floating about not doing anything and then just started to move apart. It wasn’t an explosion that caused the increasing distance apart but the fact that the space between them was increasing.

Measuring how objects move in spacetime allows us to measure the curvature of the Universe.


The problem is that there really isn’t a correct picture at the moment. Dr Clough imagined the Universe as in the above picture with us humans like little ants crawling around the surface of the Universe trying to work out how curved it was in the past, how curved it is now and how curved it will be in the future.

The way that we try to work out how curved the Universe is by looking at how galaxies are moving apart and together (our Milky way galaxy will collide with our neighbouring galaxy Andromeda in about 4.5 billion years).


There is a nice feature of our Universe that is very useful. If we can measure things about objects very far away from us in space it actually allows us to see backwards in time because it takes time for light to reach us. Now light does travel very fast which is why we don’t notice a delay when we switch a lamp on because the lamp is relatively very close to us but it takes eight minutes for sunlight to reach us and for objects very far away it takes considerably longer. The above example tells us that it would take 2.5 million years for light from Andromeda to reach us. So, when we look at Andromeda, we are seeing it as it was 2.5 million years ago. This is very useful as Andromeda is very like our galaxy so we have some idea of what our galaxy looked like 2.5 million years ago. We not only see where we are now but we can see into the past.

The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest major galaxy to the Milky Way.


The Andromeda Galaxy with satellite galaxies M32, (centre left above the galactic nucleus) and M110, (centre left below the galaxy).

Of course, it isn’t quite that simple because there is a limit to what we can see in the past. That limit is about 14 billion light years.


We little ants cannot see forwards (shame as I could win the lottery) but we can see behind us, but only to a certain point. We can only go north (forward in time) as there is a one-way system and we can only see a little bit behind us.


We can’t see the North Pole and we can no longer see the South Pole. Could the South Pole have changed?


Could the South Pole have joined to another Earth? Could there be a bottleneck?

We can’t see all the way back to the start of the Universe. This means we can’t tell what happened there. We can extrapolate a little which shows the Universe is spherical so probably it will remain spherical but just get larger. However, we can imagine that something else could happen.

There is a solution to our problem. We know how energy makes spacetime curve.


So, you tell me what stuff is in the Universe now and I’ll tell you how curved it is.

Our Sun is a big ball of energy and it curves the spacetime around it and that is why the planets move around it. You can make your own model.

So if we know how big the Sun is, how much energy it has, we can work out how curved the space is around it.

The same argument can be applied to the Universe, but on a much bigger scale.


You tell me what stuff is in the Universe now and I’ll tell you how curved it is. And if the amount of stuff is constant, I can tell you how curved the Universe used to be and how curved it will be in the future, providing I have a big enough computer.

The problem here is that we don’t actually know how many stars, planets etc. that there are and we don’t know their masses either. There is also that not so little problem of dark matter.

Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass–energy density or about 2.241 x 10−27 kg/m3. Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think that dark matter is abundant in the universe and that it has had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with the electromagnetic field, which means it doesn’t absorb, reflect or emit electromagnetic radiation, and is therefore difficult to detect.

Also, could more stuff appear out of nowhere?

Physicists have used computers to model the behaviour of the Universe and found two major problems.

1) There is a “singularity” at the beginning of time.

The initial singularity (a single point in time) is a gravitational singularity predicted by general relativity to have existed before the Big Bang and thought to have contained all the energy and spacetime of the Universe. Although there is no direct evidence for a singularity of infinite density, the cosmic microwave background is evidence that the universe expanded from a very hot, dense state.

The cosmic microwave background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe, also known as “relic radiation”.

The Penrose–Hawking singularity theorems (after Roger Penrose and Stephen Hawking) are a set of results in general relativity that attempt to answer the question of when gravitation produces singularities.


The idea of the singularity is that when we look back in time, using our two astronauts to illustrate the process, we find that things will come together. They will get squashed into one space. The more they are squashed together the hotter they become. Everything in the Universe ends up being squashed into a single point called the singularity, a point at which space-time ends (or begins if a new Universe is born). We have no idea of what existed before the singularity.

Any theories we have about the singularity won’t really work because of the high energies and temperatures at the beginning of the Universe.


Is there any way to get away from the singularity? Unfortunately, you would need to move faster than the speed of light which is, of course, not allowed. So the Universe could not have escaped starting off at this single point. (below left)


Sir Roger Penrose OM FRS (born 8 August 1931) is an English mathematical physicist, mathematician and philosopher of science. (Above right)

Stephen William Hawking CH CBE FRS FRSA (8 January 1942 – 14 March 2018) was an English theoretical physicist, cosmologist, and author who was director of research at the Centre for Theoretical Cosmology at the University of Cambridge at the time of his death.

2) If we accept that the Universe began from a singularity then it became uniform far too early. It is an unlikely state for the Universe to start so immediately after the singularity.

Perhaps to make sense of it all you need a higher energy theory. Could string theory or M theory be the answer? Or maybe a theory of higher dimensions?

In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus, string theory is a theory of quantum gravity.

M-theory is a theory in physics that unifies all consistent versions of superstring theory.

In physics, three dimensions of space and one of time is the accepted norm. However, there are theories that attempt to unify the four fundamental forces by introducing extra dimensions/hyperspace. Most notably, superstring theory requires 10 spacetime dimensions, and originates from a more fundamental 11-dimensional theory tentatively called M-theory which subsumes five previously distinct superstring theories.

Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored.

Three of the four fundamental forces of physics are described within the framework of quantum mechanics and quantum field theory. The current understanding of the fourth force, gravity, is based on Albert Einstein’s general theory of relativity, which is formulated within the entirely different framework of classical physics.

The general theory of relativity breaks down with the high energies and temperatures at the beginning of the Universe and there is a need for a theory that goes beyond general relativity into quantum mechanics. Quantum fluctuations of spacetime played an important role. To describe these quantum effects a theory of quantum gravity is needed. On more formal grounds, one can argue that a classical system cannot consistently be coupled to a quantum one.

The field of quantum gravity is actively developing and theorists are exploring a variety of approaches to the problem of quantum gravity, the most popular approaches being string theory and loop quantum gravity. All of these approaches aim to describe the quantum behaviour of the gravitational field. This does not necessarily include unifying all fundamental interactions into a single mathematical framework. However, many approaches to quantum gravity, such as string theory, try to develop a framework that describes all fundamental forces. Such theories are often referred to as a theory of everything. Others, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces.

One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10−35 metres, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy particle accelerators. Therefore, physicists lack experimental data which could distinguish between the competing theories which have been proposed and thus thought experiment approaches are suggested as a testing tool for these theories.

In physics, the Planck length is a unit of length that is the distance light in a perfect vacuum travels in one unit of Planck time.

A Planck time unit is the time required for light to travel a distance of 1 Planck length in a vacuum, which is a time interval of approximately 5.39 x 10−44 s.

All points in the Universe are independent and they sort of choose how they want to start and it seems like they all wanted to start in the same way. The question is why?

An analogy to describe this problem is asking a group of people to pick a number between 1 and 100 and they all pick 42 (the answer to life, the Universe and everything) without communicating with each other.’s_Guide_to_the_Galaxy

Of course, it could be that you are all fans of “The Hitchhiker’s Guide to the Galaxy”’s_Guide_to_the_Galaxy

However, picking the same number without communicating with each other or knowing about a certain book is just like what we see in the Universe. That all points suddenly decided to choose 42, just like they all came out of the singularity in the same way.


The cosmic microwave background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe, also known as “relic radiation”. The CMB is faint cosmic background radiation filling all space. It is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, a sufficiently sensitive radio telescope shows a faint background noise, or glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum.

There are other problems too such as what are dark energy and dark matter? The merging of black holes in the past seems all wrong.

So, is there a solution?

Not at the moment but research is being done now.


We know now that the amount of stuff in the Universe is not constant. The LHC is able to make many new particles by colliding bunches of particles together (usually protons but sometimes heavy ions such as lead) at speeds close to the speed of light, which means they have very high energies. Hot dense matter is produced very like the conditions that occurred just after the Big bang. So, you could say that the LHC is trying to recreate the conditions close to the Big Bang. There are limits including the fact that the LHC can’t reach the Big Bang energy levels, yet, but the high energies are enough to create new particles such as the Higgs boson. At normal energies the Higgs particle is not seen and even when the energy is high enough for it to put in an appearance the Higgs is very unstable and decays into other particles very quickly.


Candidate Higgs boson events from collisions between protons in the LHC. The top event in the CMS experiment shows a decay into two photons (dashed yellow lines and green towers). The lower event in the ATLAS experiment shows a decay into four muons (red tracks) (Image: CMS/ATLAS/CERN). The announcement was made in 2012 that the ATLAS and CMS experiments had each observed a new particle in the mass region around 125 GeV. This particle was consistent with the Higgs boson.

The Large Hadron Collider (LHC) is the world’s largest and highest-energy particle collider and the largest machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres in circumference and as deep as 175 metres beneath the France–Switzerland border near Geneva.

The Higgs boson is an elementary particle in the Standard Model of particle physics, produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. This Higgs field is responsible for some fundamental particles having mass.

So, if very high energies can produce this one particle perhaps it could produce many others. Perhaps the high energies around the Big Bang produced lots of different particle not seen now and these particles affected how the Universe evolved. One of the aims of the LHC is to try and replicate the conditions and see if we can find these particles.

Let’s play the “Make the right Universe” game


Take a model of stuff in the Universe adding new exotic matter like the Higgs bosons (but with higher energy and different behaviours and properties) and include what we know is already in the Universe. Then take a model of the Universe that looks reasonable, coming out of some quantum gravity period (a singularity). Feed the information into a computer and run the program to see if we get a uniform Universe that we see now. If it comes up with a good theory then perhaps, we should think about it a bit more. If we find that it doesn’t work. We need to do some more work and perhaps get a bigger computer such as the MareNostrum supercomputer


MareNostrum is the main supercomputer in the Barcelona Supercomputing Centre. It is the most powerful supercomputer in Spain, one of thirteen supercomputers in the Spanish Supercomputing Network and one of the seven supercomputers of the European infrastructure PRACE (Partnership for Advanced Computing in Europe).

One of its jobs is to try and work out how the Universe began.

Are there any theories that work?

Is there any stuff that can be added to the early Universe that can explain the singularity or the uniform Universe?

There are two main competing theories.

Theory 1 is about inflationary cosmology



The one that most physicists like best is called inflation. That there was sone stuff in the early Universe that caused it to expand rapidly.

The above picture shows a very bumpy young Universe. If we look at one small bumpy region that suddenly expands then that bump gets stretched out and the region now looks uniform. This illustrates how inflation works and this is the theory depicted on the NASA image. The prototype of a Universe that works and one of the key reasons is that it doesn’t just predict that you can get this uniform Universe but predicts a lot of the detail of non-uniformities that can be seen images produced by satellites. Researchers, including Dr Clough, have shown that you can go from this first stage bumpy, non-uniform Universe, where everything is random to the state we now see in the Universe. A big plus point for the theory of inflation.


In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10−36 seconds after the conjectured Big Bang singularity to some time between 10−33 and 10−32 seconds after the singularity. Following the inflationary period, the universe continued to expand, but at a slower rate. The acceleration of this expansion due to dark energy began after the universe was already over 9 billion years old (~4 billion years ago).

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from supernovae measurements, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating.

However, the theory of inflation isn’t completely there yet because the model doesn’t work in all cases. How much is enough. Does making the Universe too bumpy at the start of our model and not getting the expected result mean that our model is wrong. Should the theory be thrown away? Is the mechanism robust enough to stand up to the kind of non-uniformity in the initial distribution that we expect from a kind of quantum gravity singularity origin of the Universe.

We can’t really answer that question until we know exactly what those conditions would be and so its an interplay between researchers who work on quantum gravity telling us what we should expect and other researchers telling them what kind of inflationary theory of the Universe they could live with.

Theory 2 is a challenger to theory 1 because the inflationary theory doesn’t get rid of the mysterious bright white light after the Big Bang.


The challenger to the inflation theory. which is a bit easier to understand, involves bouncing cosmologies.


What if the origin of the Universe is not a point but actually goes through a bottleneck as shown in the above picture (borrowed from Quantum magazine)? A bouncing cosmology. Recent simulations by Anna Ijjas and her collaborators have shown that this mechanism also creates a uniform Universe in the same way that inflation does and, in some ways, it is more effective and more efficient in a wider range of initial conditions.

So why isn’t this theory the accepted one. Well, for this sort of bouncing cosmology the miracle moment is the point where you go through the bottleneck (not a singularity). At the point the Universe is going from contraction to expansion and this is very hard to achieve with the normal stuff we find in the Universe. It breaks what is called the null energy condition (that the density of the Universe should go down as it size increases) and some researchers consider this a step too far. If we violate the NEC the density of the universe must grow as the universe grows—so something has gone very seriously wrong. (In particular, not even a cosmological constant will let you violate the NEC.) Note that we do not even need to know at this stage if the universe is spatially open, flat, or closed to get this conclusion.

In relativistic classical field theories of gravitation, particularly general relativity, an energy condition is one of various alternative conditions that can be applied to the matter content of the theory when it is either not possible or desirable to specify this content explicitly. The hope is then that any reasonable matter theory will satisfy this condition or at least will preserve the condition if it is satisfied by the starting conditions.

Energy conditions are not physical constraints per se, but are rather mathematically imposed boundary conditions that attempt to capture a belief that “energy should be positive”. Many energy conditions are known to not correspond to physical reality—for example, the observable effects of dark energy are well-known to violate the strong energy condition.

In general relativity, energy conditions are often used (and required) in proofs of various important theorems about black holes, such as the no hair theorem or the laws of black hole thermodynamics.

In cosmology, the cosmological constant is the energy density of space, or vacuum energy, that arises in Albert Einstein’s field equations of general relativity.

Just because there are problems with theory 2, we shouldn’t completely dismiss it as we don’t fully understand what is happening. We need to understand why either theory doesn’t completely work.

We need to narrow down the picture to a sort of core set of ideas that do work for us to focus on.

Assuming you find a type of stuff that gives you the right result for these two theories, how do you know which one is the right one?

They sound like they could both be right but in order to decide we need more data, more information.

Perhaps detecting gravitational waves can help. These allow us to see things that were not visible to us before.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These observatories use mirrors spaced four kilometres apart which are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton.

One little problem is that our current gravity wave detectors are all ground based, which means that they can experience interference, but there are plans to put one into space in 2033.



Artist’s conception of LISA spacecraft

The Laser Interferometer Space Antenna (LISA) is a mission led by the European Space Agency to detect and accurately measure gravitational waves —tiny ripples in the fabric of space-time—from astronomical sources. LISA would be the first dedicated space-based gravitational wave detector. It aims to measure gravitational waves directly by using laser interferometry. The LISA concept has a constellation of three spacecraft arranged in an equilateral triangle with sides 2.5 million km long, flying along an Earth-like heliocentric orbit. The distance between the satellites is precisely monitored to detect a passing gravitational wave.

It will have limits with its accuracy but with considerably longer arms than terrestrial detectors it will better at detecting gravitational waves that are small and originated from the early Universe.

The successors to LISA may be able to give us an earlier picture of the Universe than the uniform one that originates from the cosmic microwave background radiation. It might give us more data so we can work out what is going on during the miraculous white light and which of our two theories is most likely to be the right one (or even if there is a theory we haven’t thought of yet).

Changing the shape of the Universe changes how we see our place within it. Understanding the shape of the Universe can tell us the origin and ultimate fate of the Universe, but we have a long way to go.

We are all interested in how the Universe began, where we came from and where we are going in the future. As a teacher I found that even the least interested students found the origin of the Universe fascinating (also whether they could make a nuclear bomb in class or play with the Bunsen burners).

Questions and answers

1) If space is expanding is time expanding too?

Space and time are connected together so space-time is expanding and we are exploring that.

2) Could the Universe be curved locally but flat over a large distance?

No. All the evidence is that all the Universe is expanding.

3) The bouncing model has a cycle of accelerating and decelerating. To do this negative energy is required, which doesn’t exist.

4) The similarities between a singularity and a black hole are not well understood. The similarity increases as you go back in time. They would both cause spaghettification.


Astronaut falling into a black hole (schematic illustration of the spaghettification effect)

In astrophysics, spaghettification (sometimes referred to as the noodle effect) is the vertical stretching and horizontal compression of objects into long thin shapes (rather like spaghetti) in a very strong non-homogeneous gravitational field; it is caused by extreme tidal forces. In the most extreme cases, near black holes, the stretching is so powerful that no object can withstand it, no matter how strong its components. Within a small region the horizontal compression balances the vertical stretching so that small objects being spaghettified experience no net change in volume.

5) Quantum entanglement does not explain a uniform Universe. At a quantum level things get fuzzy.

Quantum entanglement is a physical phenomenon that occurs when a pair or group of particles is generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the pair or group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.

6) If gravity isn’t actually an attractive force why can we detect gravity waves?

LIGO is actually just detecting a change in the length of its arms.

7) If gravity isn’t a force perhaps there isn’t a graviton.

8) With the bottleneck model you will never get a singularity

9) LISA will not be limited to the curve of the Earth. It will be more sensitive measuring different frequencies.

10) Hubble expansion shows that Space-time is curved.

11) Null energy conditions involve density and pressure.

12) Would the speed of light be different in the inflationary era?

In an inflationary Universe, any two particles, beyond a tiny fraction of a second, will see the other one recede from them at speeds appearing to be faster-than-light. But the reason for this isn’t because the particles themselves are moving, but rather because the space between them is expanding. Once the particles are no longer at the same location in both space and time, they can start to experience the general relativistic effects of an expanding Universe, which — during inflation — quickly dominates the special relativistic effects of their individual motions. It’s only when we forget about general relativity and the expansion of space, and instead attribute the entirety of a distant particle’s motion to special relativity, that we trick ourselves into believing it travels faster-than-light. The Universe itself, however, is not static. Realizing that is easy. Understanding how that works is the hard part.

13) Is the Universe flattening over time?

The Universe is curved in time

The shape of the universe, in physical cosmology, is the local and global geometry of the universe. The local features of the geometry of the universe are primarily described by its curvature, whereas the topology of the universe describes general global properties of its shape as of a continuous object. The spatial curvature is related to general relativity, which describes how spacetime is curved and bent by mass and energy, while the spatial topology cannot be determined from its curvature; locally indistinguishable spaces with different topologies exist mathematically.

The observable universe can be thought of as a sphere that extends outwards from any observation point for 46.5 billion light years, going farther back in time and more redshifted the more distant away one looks. Ideally, one can continue to look back all the way to the Big Bang; in practice, however, the farthest away one can look using light and other electromagnetic radiation is the cosmic microwave background (CMB), as anything past that was opaque. Experimental investigations show that the observable universe is very close to isotropic and homogeneous.

In physics, redshift is a phenomenon where electromagnetic radiation (such as light) from an object undergoes an increase in wavelength. Whether or not the radiation is visible, “redshift” means an increase in wavelength, equivalent to a decrease in wave frequency and photon energy, in accordance with, respectively, the wave and quantum theories of light.

Neither the emitted nor perceived light is necessarily red; instead, the term refers to the human perception of longer wavelengths as red, which is at the section of the visible spectrum with the longest wavelengths.

There are three main causes of redshifts in astronomy and cosmology:

a) Objects move apart (or closer together) in space. This is an example of the Doppler effect.

b) Space itself is expanding, causing objects to become separated without changing their positions in space. This is known as cosmological redshift. All sufficiently distant light sources (generally more than a few million light-years away) show redshift corresponding to the rate of increase in their distance from Earth, known as Hubble’s law.

c) Gravitational redshift is a relativistic effect observed due to strong gravitational fields, which distort spacetime and exert a force on light and other particles.

14) Gravitational bound systems will not move apart so the two astronauts used to illustrate curved space-time would have to be very far apart and in completely empty space.

15) Can we use telescope to see the Universe expand?

No. The only real tool is to look at the amount of red shift.

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