The Universe: Half an Hour at a Time

Questions and Answers

Listed here are some of the questions that were asked before, during or after the short talks given in August 2021. If the questions were not answered during the chat sessions after each talk (due to time constraints or if the answers referred to web links) then the questions and answers are given here.

These icons are used to indicate the topic(s) covered by each question:

Stars Galaxies Black Holes
Dark Matter
 Big Bang

Miscellaneous questions not directly related to the talks

If a galaxy is said to be 100 million light-years away, does that mean its distance now or its distance 100 million years ago?

It could be used to mean either distance. For distances less than about a billion light-years, the difference is only a few percent and so it doesn't really matter. For greater distances, it does start to matter (as the Universe expands a significant amount in the time it takes the light to travel from the galaxy to us). If a distance is not specified explicitly as the distance now or the distance when the light was emitted, then it is common practice to mean the distance that the light has travelled to reach us.

I saw a photograph of white aurora. Would this be fake or real? I have never heard of such a phenomenon.

Aurora result from energetic particles emitted by the Sun striking the Earth's atmosphere and exciting the atoms. As a result of this excitation oxygen emits red and green light, whereas nitrogen emits blue light. Green tends to dominate, but when solar activity is high all colours might appear and combine to produce pale yellow or pink hues. Also, auroral displays above thin cloud may appear rather pale and give the appearance of white aurora.

I have read that the asteroid Apophis will hit the Earth. When will that happen and how much damage will it cause?

Apophis will pass close to the Earth in 2029 and during that close approach it will get nudged by the Earth's gravity into a slightly different orbit around the Sun. That nudge MIGHT put it on a collision course with the Earth in 2036, but recent calculations indicate that this is very unlikely. Millions of tons of rock travelling at over 10 km/s can do a lot of damage if it hits the Earth – it would make a crater many km in diameter.  [More from NASA]

If the Moon is slowly getting further away from the Earth, when will the last total eclipse of the Sun happen?

The average distance from the Earth to the Moon is increasing by a few cm every year. Hence the apparent size of the Moon in our skies will slowly decrease and at some point in time it will no longer appear large enough to cover the Sun's disc in a total solar eclipse. (The average distance from the Earth to the Sun is also increasing, but because the Earth-Sun distance is 400 times larger than the Earth-Moon distance, the apparent size of the Sun will not change significantly.) At the rate at which the Moon is moving away from us, total eclipses of the Sun will continue for another billion years or so.  [More from]

Questions asked during

How will data from the James Webb Space Telescope be transmitted to Earth from the Lagrange point despite being eclipsed by the moon?

Approximate alignments of the Earth, Moon and the L2 Lagrange point will occur every month, near to Full Moon. The JWST will be in a 'halo' orbit around the L2 point, and exact alignment of the Earth, Moon and JWST will not occur very often. The same is true for other spacecraft that are 'parked' at L2, like Gaia.

I have heard that the James Webb Space Telescope can almost go close to seeing the big bang event itself. Do you think so? Is it even possible to see the big bang now?

No telescope can 'see' further back than the time at which the Universe became transparent, which happened about 380,000 years after the Big Bang. The visible light from this era has been stretched by the expansion of the Universe and so is now detected as microwaves – the Cosmic Microwave Background. The Hubble Space Telescope can see about 13 billion years back in time and it is expected that the JWST will see even further back, to perhaps 13.5 billion years. These 'lookback' times are getting closer to the age of the Universe (13.8 billion years) but will never actually reach it.

If galaxies are receding at more than the speed of light, can we detect them?

Yes. See the bottom of the web links page where there are links to a talk that I have given describing how I imaged a quasar that is receding from us at twice the speed of light.

Did I understand correctly that you said the universe is expanding faster than the speed of light (with distant galaxies moving away faster than the speed of light)?

Yes, that is correct. The recession speed of distant galaxies is proportional to their distance and the ratio of speed/distance is the Hubble constant. Its current value is 70 km/s for every Mpc of distance (1 Mpc = 3 million light-years). Hence galaxies beyond a certain distance are receding from us at speeds greater than the speed of light.

See also the related questions immediately above and below.

Why is the Helix Nebula not fully symmetric? Isn't a star nearly perfectly symmetric, so upon explosion shouldn't the result remain symmetric?

The planetary nebulae that can form at the end of a star's life can have multiple layers, or shells, that are sculptured by stellar winds. These winds can be influenced not only by small (possibly undetected) companion stars in multiple-star systems, but also by planets orbiting the star. Planetary nebulae change their morphology over time, and so should be thought of as ephemeral structures.

Can we pin point the centre of the universe yet?

The Universe has no centre. The Big Bang created space, and so happened 'everywhere'. There is nothing special about our location within the Universe and, we assume, the same is true of any location. (See the similar question 'Can we identify where in space the Big Bang happened?' below.)

Questions asked in advance of

What missions will the Nancy Grace Roman Space Telescope be able to do that the Webb cannot?

The Nancy Grace Roman Space Telescope was formerly known as the Wide-Field Infrared Survey Telescope (WFIRST). The old name tells you that this telescope will be a wide-field survey telescope, which makes it complementary to the James Webb Space Telescope (JWST) which has a narrow field of view designed to study specific objects in detail.  [More from NASA]

How did they make the JWST mirror so much larger than Hubble's, yet significantly less massive?

The JWST mirror comprises 18 separate hexagonal segments. Each segment is 1.3 m in 'diameter' and so is a quarter of the area of the Hubble mirror. Hence they can be made much thinner, and by making them out of beryllium (which is less dense than aluminium or glass) there is an overall mass saving of a factor of ten.

What advantage does the gold coating of the Hubble's beryllium mirror provide?

I presume this is asking about the JWST mirror, as Hubble's mirror is made of glass. A gold coating about 100 nm thick was applied to the JWST mirror to improve the reflectivity of shorter infrared wavelengths.

Why are the advantages accrued from the [JWST] Lagrange point location worth the 'gamble' that it won't need servicing?

The JWST works in the infrared and so cannot operate close to the Earth or the Moon without needing cryogenic cooling of its instruments, which would limit its operational lifetime. Placing it at the L2 Lagrange point and using a sunshield keeps the telescope and its instruments cold.

The JWST will have two spectrographs, the near-infrared and the slitless. How will they differ in their observations?

Actually it has three infrared (IR) spectrographs; two working in the near-IR (wavelengths < 5 µm) and one working in the mid-IR (wavelengths > 5 µm). The two near-IR spectrographs are NIRSpec and NIRISS. NIRSpec can record spectra from single objects by passing the light through a slit, whereas NIRISS is a slitless spectrograph that can record low-resolution spectra of multiple objects in its wide field of view.  [More from NASA]

If you were allowed to choose, what would be your first target for the JWST and why?

The JWST will have plenty of targets on its 'to do' list. My choice for the first target would be a Webb Deep Field. Distant galaxies have their light redshifted due to the expansion of the Universe, and the most distant galaxies will have their ultraviolet and visible light redshifted into the infrared region of the spectrum where the JWST will operate. Hence these galaxies should appear brighter in the infrared and so the JWST will be able to see fainter, more distant galaxies, and hence see further back in time, than the Hubble Space Telescope. This will give us valuable information about the early Universe.  [More from NASA]

Questions asked during

Is it possible that primordial black holes are still around?

Black holes created in the early Universe may still be around today providing that they have a large enough mass. A black hole with a mass of a thousand tons would evaporate through Hawking radiation in a few minutes. Those with a mass of a million tons would last a few thousand years. Still too short-lived. Primordial black holes with a mass of a billion tons or more would evaporate slowly enough for some of them to still exist today. It is not known whether such primordial black holes exist and whether they contribute to the dark matter content of the Universe.

Presumably for a gravitational wave detector the longer the base line the more sensitive the instrument. I believe there's a plan to put something like this into space ...

A large space-based laser interferometer (LISA) is being planned because it will be sensitive to gravitational waves with much longer wavelengths. These are expected to originate from objects significantly larger than the black holes that have been observed so far by Earth-based observatories, such as LIGO. It is hoped that LISA will be able to detect objects like supermassive black holes.
... (presumably at a Lagrange point) ...

Current plans for LISA have three satellites in a triangular 'constellation' bouncing lasers back and forth between them. The constellation will be in an Earth-trailing orbit about 20 degrees behind the Earth, placing it between Earth and the L5 Lagrange point.
... to provide a longer baseline than can be achieved on Earth. What separation of laser and mirror might we achieve, and when?

The planned separation of the satellites will be more than a million km. In 2015 a 'LISA Pathfinder' mission was launched to test the technology necessary for the full LISA mission proposed for the 2030s.  [More about LISA]

When two black holes collide how do you know at what distance that took place?

Although it might look like just a few wiggles, the signal detected by a gravitational wave observatory has a time structure that depends on the masses of the individual black holes that merged, their orbits around each other, the mass of the resultant black hole, and their distance from us. Each of these parameters produces subtle differences in the signal waveform and careful analysis is required to tease out the numbers for any given waveform. For instance, the first LIGO detection (GW150914) was determined to be two black holes (mass = 29 and 36 solar masses) merging into one (mass = 62 solar masses) at a distance of 400 Mpc (1.3 billion ly). The distance can be calculated from the amount of energy released as gravitational waves (the energy equivalent of 29 + 36 – 62 = 3 solar masses) and the observed amplitude of the waves.

Are there "rogue" black holes hurtling through the universe?

There have been reports of blacks holes "wandering aimlessly" through the galaxy or "rogue" black holes zooming or careening or hurtling around. These reports imply that all other black holes move around with a sense of purpose, or that somehow these particular black holes have villainous intent. Using descriptions like these is simply a way of persuading you to read the report (and in the case of web pages, the adverts as well). Stars can gravitationally interact with each other and be ejected from star clusters, or even their parent galaxy, and there is observational evidence for such stars existing in between galaxies. There's no reason to expect that black holes can't do the same.

What is the minimum size of a black hole that is still stable?

According to Stephen Hawking, all black holes emit what is now called Hawking radiation. This radiation originates just outside the event horizon, not inside the black hole itself. This loss of energy is equivalent to a loss of mass and so, given enough time, all black holes will evaporate. Small black holes evaporate quickly; supermassive black holes take a long time (see the related question below). However, no black hole will last forever and hence none of them are stable.

Questions asked in advance of

Is the radius of a black hole's event horizon proportional to its mass?

Yes. A black hole with the mass of our Sun would have an event horizon with a radius of a few km. For a supermassive black hole like M87*, with a mass of billions of solar masses, the radius is about the size of our solar system.

Do scientists actually believe that there is a singularity inside a black hole, or is it just that this is a mathematical prediction of current theories, but they could be wrong/incomplete?

A singularity is the result of gravitational collapse that cannot be halted – compressing a finite mass into an infinitely small space, resulting in an infinite density. At present we know of nothing that can get in the way of a singularity forming, but of course our knowledge could indeed be incomplete. Some have hypothesised that somehow quantum mechanics will intervene to avoid the infinite density associated with a singularity, but so far no one has constructed a theory in which gravity and quantum mechanics are good bedfellows.

What are black hole jets composed of?

Theories about how the jets seen emerging from black holes are produced have been around since the 1960s and 1970s, but there is still very little observational evidence to test these theories. The jets are thought to comprise highly energetic particles of matter and antimatter, perhaps electrons and positrons (anti-electrons). The magnetic fields near the poles of a black hole are very intense and very chaotic, and astrophysicists are carrying out computer simulations to see if these fields can accelerate matter to the energies required to create jets that can travel up to millions of light-years.

Is the universe inside a white hole?

White holes are the hypothetical 'opposites' of black holes. Anything inside the event horizon of a black hole cannot get out, whereas anything outside the event horizon of a white hole cannot get in. Science fiction writers like the idea of connecting a black hole to a white hole to make a 'wormhole' that provides a shortcut to connect different points in space and time. Thinking of the Universe as a white hole implies that nothing from outside the Universe can get in, which invokes the problem of defining what we mean by "outside the Universe".

Will we ever be able to explore inside a black hole?

There's no problem with the idea of going through the event horizon of a black hole to explore inside. The catch is that any person or probe that goes in can never come out. Also, any signal sent by the heroic explorer cannot escape. Hence, whatever you learn cannot be communicated to anybody outside the black hole.

Did a black hole give birth to the Universe?

There are no lack of theories describing how the Universe might have come into existence. Unfortunately, very few of them can explain why our Universe is the way it is, with predictions that can be tested against observations. The Big Bang theory is one such theory that has, for the past century, been able to explain many observed phenomena.

It has been hypothesised that the extreme conditions at the centre of a black hole can give birth to a new 'daughter' universe. This new universe would have its own space and its own time and would be cut off from its 'mother' universe. Perhaps our Universe was born this way? Perhaps. The problem is that this hypothesis can be neither proved nor disproved.

At the end of time will the universe consist of dark matter, dark energy and black holes (if the galaxies are gobbled up by their 'parent' black hole)?

The fate of the Universe far, far into the future is not certain. Dark matter and dark energy have been given the labels 'dark' for a good reason – we don't understand what they are and hence how they will influence the Universe in the future. Putting those aside for the moment, we can say that star formation will end in a few trillion years when all the hydrogen has been used up. Even the smallest of stars, the most long-lived, will die a few trillion years after that. All that will be left is black holes and photons of radiation. Eventually, all the black holes will evaporate. The largest, the supermassive black holes, evaporate very slowly and so it might take a googol years for this to happen (1 googol = 10 to the power 100).

Bringing dark energy back into the mix, if it continues to increase the way it is doing at present (which is by no means certain) then the expansion of the Universe will continue to accelerate. This expansion will rip apart galaxy clusters, and then individual galaxies, and then stars, and eventually all matter. One way or the other, the far future of the Universe looks much less interesting than the present.

"Eternity is a very long time, especially towards the end."

Questions asked during

What happened three minutes before the big bang?

The Big Bang is thought to be the origin of space and time. Hence there was no 'before'. It is like asking the question, on the Earth what is further North than the North Pole? It cannot be answered because when you are at the North Pole, all directions point South.

Do the models describe the behavior of the dark matter? Are there dark matter structures, or is it homogeneous?

Simulations indicate that dark matter collapsed to form the cosmic web of filaments and voids, and then the normal matter was gravitationally attracted to the densest regions of dark matter. Hence, on the largest cosmological scales, dark matter and normal matter have similar distributions. On galactic scales they start to behave differently because they have fundamentally different properties. For instance, dark matter does not collapse to make stars (see the similar question below, asked in advance of the talk). On scales much smaller than a galaxy it is assumed that dark matter is essentially homogeneous.

Are the supermassive black holes at the center of galaxies thought to comprise mostly dark matter or ordinary matter?

This is one of the questions that simulations may be able to address. Some SMBHs existed only one billion years after the Big Bang. How did they grow so fast? Conventional theories say that normal matter collapsed into stars, some of which formed black holes, and then these grew over time. Alternatively, it might be that dark matter collapsed into SMBHs before the rest of the galaxies formed. This is an active area of research.

Is it possible that even heavier "unknown elements" exist elsewhere in the universe?

The heaviest naturally occurring element is uranium (atomic number 92). In this context, 'natural' means having a half-life comparable to the age of the Earth, so that if such an element existed when the Earth was formed then there is still some of it around today. Heavier elements have been made in research laboratories up to atomic number 118, but these elements are very unstable and decay in a matter of seconds or even milliseconds. It is hypothesised that there may be some heavy elements that we have not yet been able to synthesise that are stable. If this is the case, then they may have been created in a supernova and hence exist somewhere in the Universe.

When a star dies, does the type of star it is determine if it will become a black hole or not?

The ultimate fate of any star is determined by its mass. A star with a mass comparable to that of our Sun will ends its life as a white dwarf. A star with a mass of about 10 times the mass of our Sun will probably end its life in a supernova and the core will be compressed into a neutron star. For a more massive star, about 20 times the mass of our Sun, the supernova will create a black hole.

Do the simulations take account of gravitational waves?

As best I can tell the simulations I was describing would not have the creation of gravitational waves built into them. If the simulation indicated that black holes of a given size might merge at a given rate, then that information could be used in a separate simulation of black hole mergers to predict what the resultant gravitational waves would look like. This in turn could be used as a 'signature' to look for with a gravitational wave observatory such as LIGO.  [More about LIGO]

Questions asked in advance of

Is there a correlation between the size of a galaxy and the size of its supermassive black hole?

In general, the bigger the galaxy, the bigger the SMBH at its centre. The strongest correlation is found with the mass of a galaxy's central bulge, rather than the total mass of the galaxy (central bulge plus disk). Plotting the mass of a SMBH versus the mass of the host galaxy's central bulge gives this plot. There is quite a bit of scatter in the data, but the correlation is clear.

The Milky Way has an outer rim of stars orbiting in the opposite direction to the rest of galaxy — is this evidence of a past galaxy merger? Do other galaxies have this feature?

Counter-rotating stars (and gas) have been observed in many galaxies. The ESA mission Gaia is making a three-dimensional map of the Milky Way by measuring the positions of stars very accurately (to a precision measured in microarcseconds). Gaia has found stars in very eccentric orbits, and counter-rotating orbits, and this data is being used to help understand the evolution of the Milky Way due to galaxy collisions and mergers.  [More about Gaia]

Is there a correlation between the amount of dark matter and the amount of visible matter in a galaxy? Does this depend on the size of the galaxy?

It appears that all galaxies contain some dark matter, but estimates of the fraction of a galaxy's mass that is dark matter can vary significantly from one galaxy to another. The ratio of dark matter to normal matter averaged across the Universe is about 5:1 and so it is assumed that all galaxies start their lives with this mix. Stars produce radiation that interacts with normal matter (but not dark matter) and this can result in normal matter being expelled from a galaxy. Small (dwarf) galaxies in particular find it difficult to hold on to the matter and hence, over time, the ratio of dark matter to normal matter can increase. For the smallest of galaxies the ratio can exceed 100:1.

When the Milky Way and the Andromeda Galaxy merger occurs, will the resultant galaxy exist with two supermassive black holes or will those SMBHs eventually merge?

In about four billion years the two galaxies will collide, pass through each other and, after another billion years or so, merge into one huge galaxy. At that time the two SMBHs will still be separate bodies orbiting around one another. When they eventually merge it is likely that huge amounts of matter will fall in to the new SMBH and the new galaxy ("Milkdromeda"?) could develop an active galactic nucleus, and perhaps even become a quasar.  [More from NASA]

Dark matter and normal matter are both affected by gravity. Normal matter clumps into stars, planets, etc. Does dark matter do the same and if not, why not?

We know that dark matter does clump together – these are the places where galaxies form. Making stars is different. When normal matter collapses under gravity the particles radiate heat and cool down, allowing them to concentrate into a smaller volume and eventually form a star. If dark matter tries to do the same thing, the dark matter particles can't cool down (as they do not interact through the electromagnetic force) so they just 'pass by each other' and do not concentrate into a small volume.

Note that a 'dark star' is NOT a star made from dark matter. The term is used to describe a star that is almost entirely hydrogen but with a tiny fraction of dark matter in the mix. These hypothetical stars are thought to have existed in the early Universe.  [More from BBC]

Questions asked during

Where does electromagnetism come into all of this?

In the very early Universe nuclear forces determine how quarks stick together to make protons and neutrons and also how protons and neutrons end up as the nuclei of hydrogen and helium atoms. When the Universe is cold enough (a balmy 3000 K) the positively charged nuclei can hang on to the negatively charged electrons that have been swimming around in the soup of particles. It is electromagnetic forces that are responsible for making atoms. Now that we have electrically neutral atoms, the dominant force switches over to gravity which then shapes the large-scale structures of the Universe. When the first stars are born, millions of years after the Big Bang, their intense radiation can ionise (strip the electrons from) atoms. Electromagnetic forces then have a role to play in further star formation.

Is ALL of space expanding, including the space in between our atoms and everything else we see?

As the Universe expands galaxies tend to get 'carried along for the ride' and hence are observed to be receding from us. This is called the Hubble flow. Smaller objects, like the atoms and molecules in our bodies, only care about the electromagnetic forces that result from positive and negative electric charges in the atoms. They are essentially unaware of what the Universe is doing. Solar system objects like the Earth only care about the big gravitational pull of the Sun. The Sun feels the pull of the rest of the galaxy as it makes its slow orbit around the Milky Way. Isolated galaxies that do not feel the strong pull of their immediate neighbours will 'go with the flow' and get carried along by the expansion of the Universe.

Is the age of the Universe (13.8 billion years) the inverse of the Hubble constant?

Yes. The Hubble constant tells us how fast a galaxy at a given distance is receding from us (speed divided by distance). If we calculate the reciprocal of that number (distance divided by speed) that tells us how long it would have taken for the galaxy to reach that distance if it travelled at that speed. We can equate this with the age of the Universe. The only complication is that the Universe has not been expanding at the same rate for all of its existence. In other words, the Hubble 'constant' used to have a higher value. If we take that into account, then the reciprocal of the Hubble constant is roughly equal to the age of the Universe.

When the observable Universe was size of a golf ball, did the matter near the outside boundary of that golf ball have a way to recognize that it was near the boundary?

The observable Universe is a sphere that is currently about 46 billion light-years in radius. It is better to think of the surface of that sphere as a horizon, rather than a boundary. Just as for a horizon seen by a ship at sea, we can see objects closer than the horizon and we cannot see objects beyond the horizon. There is no physical boundary as such. It is reasonable to assume that the Universe (the sea) is probably much the same beyond the horizon even if we cannot see it. There is nothing special about the centre of the sphere, except that this is where we happen to be. (What is at the centre of a ship's horizon? The ship.)

It is hypothesised that a tiny fraction of a second after the Big Bang the sphere that we currently describe as our observable Universe was about the size of a golf ball. Within the volume of the golf ball was all the matter that would, eventually, evolve into the stars and galaxies that we see today. Note that this does not imply that the entire Universe was no bigger than a golf ball – we do not know the extent of the Universe beyond our observable Universe, by definition. It may be finite, it may be infinite.

Why can we not see before the first picosecond?

We cannot see anything that happened before the Universe became transparent, because it was only then that light was able to travel unimpeded throughout the Universe. That 'first light' was released about 380,000 years after the Big Bang and is now observed as the cosmic microwave background.

Prior to that, we have to rely on theory. The Big Bang theory is a model that explains the evolution of the Universe from the first tiny fraction of a second to the present. Predictions of what the Universe looked like a picosecond after the Big Bang can be tested by reproducing the conditions, in a limited way, in the laboratory. Prior to a picosecond, we cannot test the theory against any observational evidence and so any statements about the Universe remain unsubstantiated. Hence, a picosecond is the watershed that separates what we know from what we can only hypothesise.

Can gravitational waves be used to detect the moments of the very earliest universe?

In principle, yes, but not with the gravitational wave detectors that exist at present. The current generation of gravitational wave detectors are laser interferometers that have arms that are about 4 km in length. This makes them sensitive to gravitational waves that result from mergers of black holes and neutron stars, but not the 'primordial' waves resulting from the Big Bang. Detection of these primordial waves would require a laser interferometer with arms millions of km in length. There are plans to build such an interferometer.  [More about LISA]

How can the expansion of the Universe happen at a speed faster than the speed of light?

Nothing can move through space faster than the speed of light, but space can expand at any speed it likes. As space expands the galaxies get carried along like corks caught in a water current. They are not moving THROUGH space, just as the corks are not moving through the water, and so no laws of physics are violated.

Questions asked in advance of

Is the Universe cyclic, a series of expansions and contractions?

That was considered a possibility until a couple of decades ago when it was discovered that enigmatic dark energy is accelerating the expansion of the Universe. It looks like gravity will not be able to overcome this expansion and bring about a contraction, so the Universe will continue to expand forever.

If all galaxies in the universe should be accelerating away from us, how can the Milky Way and the Andromeda Galaxy be on a collision course?

The Universe has been expanding ever since its origins in the Big Bang. As it expands galaxies tend to get 'carried along for the ride' and hence are observed to be receding from us. In addition to this effect, called the Hubble flow, an individual galaxy might get pulled around by the gravity of nearby neighbouring galaxies. At large distances from us, the recession velocity of a galaxy is so high that the gravitational pull of its neighbours makes little difference. However, the Andromeda Galaxy is so close to us that the gravitational attraction of the Milky Way and the Andromeda Galaxy is strong enough to dominate over the effect of the Hubble flow. In about four billion years the two galaxies will collide, pass through each other and eventually merge into one huge galaxy.  [More from NASA]

Is the universe infinite?

A big question in just four words. An answer in three: We don't know.

The observable Universe is finite – a sphere that is now about 46 billion light-years in radius. Any objects outside this sphere cannot be seen by us because any light they emitted, even light emitted long ago when the Universe was very young, has not yet had time to reach us. Outside this sphere the Universe might look much the same as it does inside, but we don't know because we have no observational evidence.

If space is infinite, where is the centre of infinity?

The concept of a 'centre' has no meaning for something that is infinite in extent. Another point to bear in mind is that even something that is finite does not necessarily have a centre. For instance, consider the two-dimensional surface of the Earth. Despite having a finite area it has no boundary (you cannot travel on the surface to the 'edge of the world') and no centre.

If the universe is infinite but the amount of matter within it is finite, why was it so surprising to find that the universe expansion rate is increasing?

The Universe might be infinite, but if it is then there is no reason to assume that the matter within it is finite. If this was the case, then the distribution of matter outside the observable Universe would have to be very different from what we can see inside it. This goes against the 'cosmological principle' which assumes that the distribution of matter in the Universe is the same everywhere. In other words, we do not live in a 'special' place within the Universe.
As space is expanding, the amount of space that is receding from us at greater than the speed of light is similarly increasing. Thus the gravity of the matter in that space can no longer have any effect in our 'local' universe to resist that expansion.

There's an assumption here that objects receding from us at a speed that is greater than the speed of light cannot influence us or be seen by us. That's not the case. For instance, I have photographed a galaxy (without using a telescope) that is receding from us at about twice the speed of light – more details are given in my talk Ancient Light.

Can we identify where in space the Big Bang happened?

The Big Bang was the origin of everything, including space itself. Hence we should not think of space as existing first and then imagine watching the Big Bang happen at a particular point in space. It is better to think of the Big Bang as happening everywhere.

What is space expanding into?

It's not expanding INTO anything, except itself. One way to visualise expanding space is to think about a two-dimensional analogy – the surface of a balloon. Forget about whatever is inside or outside the balloon; just think about the surface itself. As the balloon is inflated the surface stretches and the area increases. If dots (galaxies) are painted onto the surface, they all move apart from each other as the balloon inflates. None of the dots are at the 'centre', but from the perspective of any given dot all the other dots seem to be moving away from it. That's why we see galaxies moving away from us as the Universe expands.

How does an understanding of the cosmic microwave background help us to confirm the expansion rate of the observable universe?

The cosmic microwave background (CMB) is almost isotropic, meaning we measure the same intensity wherever we look in the sky. However, there are tiny fluctuations in intensity (one part in ten thousand) that were 'imprinted' shortly after the Big Bang. Studying these fluctuations in detail allows us to probe the early Universe. Various parameters – such as the dark matter or dark energy content of the Universe and how fast it is expanding (the Hubble parameter) – all have an effect on the pattern of the CMB fluctuations. They are not easy to disentangle, so extracting the parameters requires fitting the CMB observations to a mathematical model.  [More from ESA]

Steve Barrett     August 2021
Back to  The Universe: Half an Hour at a Time

Click here for Q&A | web links | more talks
Back to top