Galaxies and Cosmology, Chapter 1
Module 1.1 - Cosmology vs. Cosmetology
Welcome to the Coursera class on galaxies and cosmology. My name is George Djorgovski, and
I'm a professor at California Institute of Technology. This class is offered to the second year
physics and astronomy students at CalTech. Therefore, it requires a certain amount of
preparation, and we'll come to that in a moment. But first of all, let us define, what is
Cosmology? It is the science of the universe as a whole, and its major constituents. How they
work, and how they evolve, how they are formed. It is different from cosmetology, and if you
made that mistake, this will be a really good time to stop. Well, it's not an easy task. Universe is
very big, and we cannot reach it all. We can just sit here and watch. And so we have to make
some assumptions. The basic assumption that we make is that laws of physics are same
everywhere and at all times. This is a very reasonable assumption. And if it weren't true, it would
be very hard to do any kind of science. But we can actually test some aspects of this proposition.
Moreover, sometimes, while studying the universe, we discover things in physics that we didn't
know about. Notable examples are nature and existence of dark matter and dark energy and we'll
talk about them later in the class. Unlike most other sciences, cosmology has only one object to
study. There is only one universe in which we live. And all we can do is sit and watch or, in
technical terms, we can look at the surface of the past light cone. So there're parts of the universe
that are a priori, not observable class. If we're pretty much sure that they can do exist, then
they're probably not very different from the one parts that we do see. But it's something to keep
in mind. And also, things are far away and, therefore, or they're faint and small and we need
most advanced technology to do this. This is why cosmology really flourished as a science in the
latter part of the twentieth century, because before then, we simply did not have good enough
tools to do it right. But even so, we have to be always aware of possible biases and selection
effects, for example there could be some very faint galaxy that we're missing.
Cosmology responds to the basic human need to understand, the big picture. Where's it all
coming from? How does it work? Where's it going? And it eh, evolved through time. In prescientific days, creation myths were made to answer these questions. But they really were just
made up stories. Starting from beginnings of modern science in the Renaissance and the
enlightenment. Things started to get a little more rigorous. First, the Copernican revolution,
which was a major shift in the way people think about universe or the world. It's positive that
we're not at a special place. The Earth is not the center of the universe. It's just a random spot.
Cosmology actually uses this in so called cosmological principle that we'll define in a bit.
But not only that. Universe is not static and unchanging. We see the change. And just like
biology in planet Earth, universe evolves on its own. And following laws of physics, major
constituents of the universe, galaxies, stars in them also evolve. And we can understand how that
works. And just like anything else in science, it became subject of study and, therefore, it is
always improvable. We're not declaring anything with absolute certainty. But we're actually
trying to understand what's going on. And like any other science, other science, the more we
push further, the more we learn, the more new questions we open up. And that's the nature of
science and this is very good. So, whereas, really, cosmology started as a branch of astronomy,
astronomy as a whole. And certainly, cosmology became a branch of physics because we use,
laws of physics and principles of physics to obtain our measurements, to understand them, to
interpret them, make predictions.
Alright. Now let us find out how well prepared you are for this class. First we'll do a few
quizzes.
Let us now resume our journey through the universe and dust off some of the astronomical units
of measurement. The most basic unit in astronomy and cosmology is distance to from the sun to
the earth. And that's one astronomical unit, and it's about 150 million kilometers. There is, of
course, the light year, which is the distance that light travels over one year. And one light year is
approximately 10 to the 18 centimeters. Now, astronomers almost never use light years. We use
parsecs, which is the distance from which one astronomical unit is seen at an angle of one
arcsecond. And that turns out to be roughly 200,000 astronomical units. Or 3 times 10 to the 18
centimeters. So when we talk about parsecs or kiloparsecs and megaparsecs and gigaparsecs, you
can multiply that by 3.26 to get number, corresponding number in light years.
Two basic properties of objects that we need to understand are masses and luminosities. And, for
convenience and historical reasons, we use solar mass as a unit and that is approximately 2 times
10 to the 33 grams, and solar luminosity, which is close to 4 times 10 to the 33 ergs per second.
So you can convert into other units as you need to. You should really remember these numbers,
because we'll constantly refer to distances, and masses, and luminosities using these units.
Now, what we observe are fluxes. We mostly observe electromagnetic radiation, although we
now have forms of astronomy that are not dependent, electromagnetic radiation like cosmic rays,
neutrinos and, soon enough, gravitational waves. But flux is what's measured and we have
detectors that can operate in full range of wavelengths from radio to gamma rays. And they're
usually measured over some finite bandpass. So spectral energy distribution, which is the shape
describes the spectrum of an object, is defined to be energy per unit time, per unit second, per
unit for frequency or wavelength. And we never observed that as the differential unit. Instead of
that, it's observed as some finite bandpass like an optical filter, or band within as radio
astronomy. One unit that is often used is Jansky, which is introduced by radio astronomers. And
it's, 10 to the minus 23, ergs per second, per centimeter squared, per hertz. Optical and infrared
astronomers are now beginning to use Janskys as well. Although we're typically talking about
microjanskys and nanojanskys. So this is often called flux density, and to get really, the power of
object, one has to be integrated over all bandwidth, and then multiplied by the area, of [flux].
Astronomers use magnitudes, which are a logarithmic measure of flux, defined by the formula
written here. The magnitude is minus four decibels, the minus sign tells you that the higher the
number, means the lower the flux. And because it's a log, it's a relative measure of flux, relative
to some unit flux. In log, that comes as an additive constant. Typically, flux is measured over
some finite bandpass like V band filter, centered on 5500 angstroms. And then log of that times
minus 2.5 plus a constant zero point, which I'll define in a second, gives you the actual
magnitude. If, for some reasons, you could integrate the flux over the entire spectrum, then this
would be called the bolometric magnitude.
Now magnitude zero points are another part of astronomical craziness that's unfortunately wellestablished and hard to change. For historical reasons, again, Vega, which is Alpha Lyrae, was
declared to have zero magnitude and then everything is measured relative to it. Note that because
we're talking about logarithms of fluxes, talking about ratios of fluxes, and so it's all relative to
some unit. Well, that unit's for magnitude systems is usually Vega. And unfortunately its
spectrum is not flat. It looks like this. There is also a more rational kind of magnitudes that's
been introduced. It's called AB new magnitudes, which are with the fixed zero points. As the
formula in the bottom shows. But typically, people use Vega based magnitudes, and it's a handy
way to remember it, that zeroth magnitude star corresponds to almost 1,000 photons per square
centimeter per second per angstrom, then you can scale from there.
Now, magnitudes are measured as apparent magnitude. What we'd like to do is know how
luminous an object really is and, therefore, we need to know how far it is. So an absolute
magnitude was introduced, which is the apparent magnitude the object would have if you
observe it from a distance of ten parsecs. What, why ten and not one, is anybody's guess, but, this
is the definition. And so if you put sun, or sun-like star, ten parsecs away from us, it will have
apparent magnitude of roughly, plus five, different in different bands, because the spectrum
shape is different from that one of Vega. So, as a handy measure of the distance, sometimes we
use the difference of apparent and absolute magnitude, which is called the distance modules. And
it's equal to five times log of the distance divided by the ten parsecs. So this is how you can
convert this. Alright, so this is it for now and the next time we'll start talking about the history of
Cosmology from earlier days to the present time.
Module 1.2 - Early History
Hello, again. Let us now review briefly the history of modern cosmology. Before we could study
the universe as a whole in scientific sense, we actually discovered and understood galaxies which
are its major constituents. At first, people didn't know what they were. They were just as
smudges in the sky, they were called nebulae, and famous catalog by Charles Messier. And then,
William Herschel also cataloged many thousands of them, and followed by others.
Now even before that was really properly understood, philosophers tried to address the question.
And they talked about island universes, or what today we would call a galaxy. They had no
scientific reason to believe one way or the other but it was an interesting speculation. Now, in
19th and 20th century, more catalogs or galaxies were produced but there was still no
understanding. For example, the very basic question was, are these nebulae, Galaxies like the
Milky Way or are there just some smudges within our own galaxy, the Milky Way, which is the
universe as a whole?
So, up until 1920s, the early 1920s, this was still an unsolved question. And there was a famous
great debate between two astronomers, Harlow Shapley from Harvard, and Heber Curtis from
Lick Observatory, whether or not galaxies or a nebula, I should say, are island universes just like
the Milky Way, or are there just some smudges of gas inside the Milky Way? The debate was
inconclusive. Shapley had wrong answer, that Milky Way is all of it. Curtis was advocating that
galaxies are many like the Milky Way. And arguments were not really based on any solid
experimental data.
This all changed with Edwin Hubble in 1923 on using Mt. Wilson Observatory. He obtained
photographs of Andromeda galaxy and found a variable star, so-called surface in Andromeda.
Comparing its brightness to brightness of surface in our galaxy, immediately told us that
Andromeda is much, much further away than anything in our galaxy. Within our galaxy, people
knew about stars that are clusters that are kiloparsecs away. Andromeda turned out to be several
hundred kiloparsecs away. Therefore, it was galaxy just like the Milky Way, and then all the
others which are much fainter, presumably even further out. So, all of a sudden, the picture
changed from Milky Way being the entire universe to a much, much bigger universe, populated
with galaxies like Milky Way and others. This is how Hubble really became famous. He did
something else later on, discovering expansion of the universe.
But, before we get into this, theory actually made some important advances. If you want to
understand universe at large from physical terms, the only interaction that actually matters is
gravity. Because all other forces are short range except for electromagnetic force, but charges are
mixed so well, positive and negative ones, so that net electromagnetic field is pretty much a zero
in any average sense. Not so with gravity. Gravity cannot be cancelled or compensated, and so, if
you want to understand how things happen on large scales, you need the theory of gravity. This
came in the form of theory of relativity, and in particular general theory of relativity.
So, Einstein came up with the way that describes that, and we'll talk a little more about it a
couple lectures from now. But he immediately understood that this can be actually applied to
study of the universe as a whole and even taught a class about that from 1919, this was before it
was actually proven that the theory was correct. He assumed that it was just normal matter like
stars and so on. But it's finite in spatial size, and we'll talk about how that's possible.
In 1917, Einstein made first cosmological models. He believed in his theory, of course, even
though it was experimentally proven only in 1919. And, he had something interesting. He, he
found out that universe has to either collapse on itself under its own gravity, or maybe expand
forever. Collapse under its own gravity seemed like a natural thing, and in order to balance that,
he introduced a force called cosmological constant. This turned out to be a very unstable
cosmological model, and it's wrong anyway. at the same time, Dutch physicist Willem De Sitter
developed similar model and obtained equations for expanding universe. Now, later on, two of
them came up with the different model that bears names but that is a different story.
Back to observations, discovery of the expanding universe was probably one of the greatest
scientific discoveries of all times. Two people need special credit here especially Vesto Melvin
Slipher who was an astronomer at Lowell Observatory in Arizona who measured velocities,
radial velocities of galaxies. And obtained the data that were much later that were later used by
Hubble. That's roughly same time Knut Lundmark and Carl Wirtz in Sweden and Germany
obtained similar results. But, their plot of velocity versus estimated distance to the galaxies didn't
show anything.
A little bit later, Edwin Hubble plotted distances to galaxies which are measured little better
using relative brightness as a measure of distance, against velocities most of which are obtained
by Slipher, and found this remarkable trend that the further away galaxy was the faster it was
going away from us. This is now known as the Hubble diagram and was immediate evidence for
an expanding universe.
Here is a brief explanation of the expansion of the universe. Imagine just a piece of the volume
of the universe populated with galaxies, more or less uniformly distributed. If the space expands
in itself, it carries the galaxies apart. And, the further apart from, from each other they are to
begin with, the larger the distances there will be next time. So, since the growth in the distance
between any two galaxies is proportional to the distance itself, and its time derivative is the
velocity. So therefore, you get the velocities of recession from galaxies from each other is
proportional to their distance which is Hubble's Law.
Now, Einstein go to see this for himself. He visited Pasadena numerous of times in 1930s. Here,
he is on Mt. Wilson with Edwin Hubble and Walter Adams, another famous astronomer, looking
though an eyepiece of the 100 inch telescope. This was completely fake and posed picture
because that's not what astronomers do. Allegedly, Einstein declared failure to predict expanding
universe as the greatest mistake of his career. And remember, he was, invented cosmological
constant as a means of preventing the collapse of the universe for that wasn't really needed. He
failed to predict expansion of the universe, even though that was implicitly contained in his
equations. And that was probably the greatest scientific prediction anyone could make. So, this is
why he wasn't so happy.
In the meantime, two theorists really developed modern relativistic cosmological models.
Alexander Friedmann in Soviet Union developed relativity-based expanding universe models.
Then, he came up with the equation that bears his name that we will use very heavily. Then,
roughly at the same time or few years later, George Lemaitre in Belgium developed similar
cosmological models. And also, he said, okay, if the universe is expanding now what if we just
look back. It all must have started with very dense state which is probably very hot and he called
this the Cosmic Egg. And so, that was essentially a first notion of the Big Bang, but it was not
taken very seriously because he was little too much of a extrapolation and he was a Jesuit priest
and you know that sounds a little suspiciously like biblical creation. So, that was forgotten for a
little while.
Then in 1930's, relativistic cosmology really started to flourish. And here are some of the more
important contributors to it. Milne developed a model that is based on special relativity. It
includes no matter. It's just the dynamics of space and time itself. And Eddington who was the
person who'd actually proven that general relativity is right promoted their own models and tried
to think about the interfaces, interfaces between quantum theory and relativistic cosmology.
Robertson and Walker are two mathematicians who developed a mathematical description that's
actually still being used in explaining or describing expanding universe.
Module 1.3 - Later History
Hello. We now continue our quick overview of the history of cosmology. In the last module, we
learned how expansion of the universe, observational discovery, and general activity which
provided a theoretical framework for cosmology, really established it as a science. Something
else however, happened in 1930 that's of considerable importance. And that is the discovery of
dark matter by Caltech physicist and astronomer Fritz Zwicky, who applied simple Newtonian
mechanics to motions of galaxies and coma cluster. Their kinetic energy has to be balanced by
the binding energy. And so he could deduce how much mass was needed in order to keep
clusters together. So he found out that he needs 400 times as much invisible matter that does
exert some gravitational influence that was actually seen in stars. Similar thing was found by
another astronomer, Sinclair Smith, for Virgo cluster a few years later. But at that time, nobody
took this seriously. Because this was just too outrageous statement and there was no other
evidence for it. Until 1970's when sufficient evidence has accumulated that dark matter could no
longer be ignored. And now, essentially, everybody believes in its existence for good reasons
which we'll cover later. Dark matter is, today a key ingredient in the models of structure
formation. But its exactly physical nature is not yet understood, and this is one of the outstanding
problems of physics today.
Meanwhile, observational astronomers following first, Hubble, but then his followers including
Allan Sandage, tried to establish what kind of relativistic cosmological model we live in. Hubble
designed a set of cosmological tests, how observations of distant galaxies can be used to
determine local geometry. The foremost of those were so-called Hubble Diagram. Remember,
that's the plot of relative distance versus velocity for galaxies far away from us. And near us, this
is very close to a straight line. Now, as you go further deeper in space, relativistic effects start
playing role and the line deviates from straight in way that depends on values of cosmological
parameters. So, Sandage and his collaborators tried very hard to do this. Using brightest cluster
galaxies is what they call standard candles, assuming that they're in general, luminosities
constant. And that basically failed, because galaxies are not constant. They evolve, they're made
of stars, stars of all the galaxies merge. And so until mid-90s, Hubble diagram was not
considered as a serious cosmological test. But then, things change considerably.
Galaxy evolution really became forefront of cosmology in 1970, or late 1970s and 1980s. And
this was in part with the realization that we must understand how constituents of galaxies evolve,
and therefore galaxies themselves before we can use them for cosmology. But it’s also
containing number of interesting question so on and so on. And, studies of this were enabled by
development of modern instrumentation, ending up in charge couple devices. And both of that
was done at Palomar Directory. Here are some of the famous astronomers who developed early
instruments to study distant universe.
Meanwhile, on the theoretical front, Fred Hoyle, Hermann Bondi and Thomas Gold came up
with a new idea called the Steady State Cosmology. Remember, the cosmological principle says
that universe is same at all places and all directions. Well, they said also at all times. But because
it expands, new matter has to be created in order to fill up the gaps. And mechanism for this was
not specified and that was clearly seen as a weakness. But in that sense, universe will always
look the same, keep expanding but always look the same. And there are cosmological tests that
can distinguish between those two. In part, this was trying to respond to the extrapolation of the
galactic cosmic expansion to what now called Big Bang which at least Hoyle found distasteful
and give it the name.
But, Big Bang Theory actually made some important predictions. George Gamow and his
colleagues, Alpher and Herman, actually considered what one Lemaitre thought was primitive
atom and ask what would be the physics if universe is so hot and dense. And so, what universe
will do is what stars do in their course, which is convert hydrogen into helium, heavier elements.
And they did develop that along with Hans Bethe and called Alpher, Bethe, and Gamow Theory.
But, it turns out that they can predict formation of elements all the way up to helium, in other
words, just hydrogen and helium. However, there's really been an afterglow of this cosmic
thermonuclear explosion which takes to the red shift stretching the photons now will not be
gamma rays, but will be really microwaves. So that black body temperature of five degrees
Kelvin or so.
Well, this was actually measured in 1965 by Penzias and Wilson, who deservedly got the Nobel
Prize for this discovery. The cosmic micro background remains as one of the touchstones of
cosmology. Today, we can measure it with space instruments, and its spectrum is as predicted
by theory, pure black body radiation with which is now measured with exquisite precision, and
might be actually the purest black body spectrum that we have measured anywhere.
Another prediction of the Big Bang Theory is that there will be abundances of very light
elements. I said, you will only make Helium, but that's not quite true. It will also make trace
amounts of Lithium and maybe a touch of Boron and Beryllium. But also, different isotopes of
Helium and Hydrogen, Deuterium, and Helium three.
So, here we have on the left plot of the helium mass fraction in star-forming galaxies, plotted
against their oxygen abundance. Oxygen is only made in stars. So if both Helium and Oxygen
were made in stars only, then this plot would be line going through the origin of zero point. But
instead of that, there is an intercept. The zero point from which it starts is 0.24 mass fraction of
Helium. And therefore, stars must begin with that much Helium, and the only place that Helium
can come from was from primordial nucleosynthesis.
Models of primordial nucleosynthesis have been developed with great precision and now they're
compared with observations. The plot on the right shows theoretical predictions as functions of
the density, Baryon density of the universe. And the blue band shows where they the measured
the value is. So, it all is consistent with what we now believe are the right cosmological
parameters. And this is also seen as one the great pieces of evidence in favor of the Big Bang
cosmology.
In the meantime, another important discovery happened, or set of discoveries really. After the
World War II, thanks to the radars, radio astronomy was born. And radio astronomers start
mapping the sky in radial wavelengths, seeing their sources with nature was at first unknown.
And astronomers like Walter Baade and Rudolph Minkowsky obtained optical counterparts,
some of these sources. And discovered some of them are actually quite far away, implying that
given the observed flux, the internal power luminosity of these objects must be enormous. This
was a very surprising and important discovery.
Given the optical, signs of something interesting were already there. In 1940's Carl Seyfert for
his PhD thesis, observed bright nuclei nearby spiral galaxies and found to have these somewhat
unusual spectra with very broad emission lines. And their nature was not understood at the time.
But it took really combination of optical and radio astronomy with identification of quasars to
really drive his point home.
Cyril Hazard was one of the radio astronomers who obtained precise measurements of few quasi
stellar sources and their positions. Allan Sandage and others at Palomar obtained their optical
counterparts, and Maarten Schmidt and his collaborators figured out what's going on.
Namely, from the shift of lines in this spectrum, they figure out they must be very far away.
Remember, the faster objects we see, the further away they are. Well, this implied enormous
distances to quasars, which then implied that the, their internal luminosities are huge. So they
have an object that may be ten times or hundred times or thousand times more luminous that
entire galaxy of stars. And that luminosity comes from regions smaller than solar system.
Maarten Schmidt made it to the cover of Time Magazine. I think he was the second astronomer
with that honor, the first one was Harold Shapley.
Meanwhile, another important line of study was happening. Namely, discovery of the large scale
structure of the universe. It really began in 1930's with Harold Shapley, Zwicky, and their
collaborators starting to map how galaxies clump in space. It was clear that they're not purely
randomly distributed. And through the 50s, the 70s, more evidence was accumulated and was
obvious that galaxies are clumped in clusters and less dense but more extensive structures.
However, the really most important new development was measurements of red shifts, which
imply distances to vast numbers of galaxies. First thousands, but now there are really hundreds
of thousands.
Gerard De Vaucouleurs, who was a famous observational astronomer, also pointed out that our
immediate extra-galactic neighborhood forms what we call the local super cluster, of which
we’re one part, and Virgo cluster was the center with some elongated structure in the sky as seen
in the projection here. And this was an indication of structures that are larger than anything seen
before.
So today, of course, we know this with a much greater precision, and here is the projected map
of density of galaxies from the Sloan Digital Sky Survey, a modern Perchet survey. And shows
these filaments and voids and bubbles which are important features of the galaxy distribution that
we will study later on.
Module 1.4 - Modern Cosmology
Let us now complete this brief survey of history of cosmology with some more modern
developments. Remember that, by 1970's, it became clear that galaxy formation evolution must
play an important role. It must be understood before we can actually map out the universe, and
both theoretical and observational advances were made. Three theorists deserve special mention.
Jim Peebles in the United States, Yakov Zel'dovich in former Soviet Union, and Martin Reese in
United Kingdom.
At the same time, we saw development of numerical cosmology. it is impossible to analytically
follow, how would systems of billions, larger numbers of particles evolve. But we can simulate
them in computer. The improvements in computer technology enabled very sophisticated and
ever better simulations to be done and here is just a set of snapshots from one by a theorist
named Andre Kravtsov.
Today this is a very well developed art, and this is just a small snapshot of one of the more
modern simulations by so called Virgo Consortium. And it covers essentially the entire
observable universe. We can also zoom in, in great detail and see how structures form on galactic
scales. So then that has to be compared with observations.
Observational cosmology really started blossoming from 1970's onward. And thanks largely to
the development of many new technologies. Both from the ground and space. This is a picture
from Hubble Space Telescope from one of the Deep Fields that Hubble has studied, with many,
many distant galaxies, but it's important to point out the important role that technology plays in
our development of scientific knowledge.
Ever better instruments is, including CCD arrays and computers, enable us to get much higher
quality of data and much more data than we ever had before. And in both large telescopes on the
ground like a twin keck telescope shown here and in space, like the hubble shown here also, play
an important role in these studies.
Another important theoretical development happened in 1980. Alan Guth came up with so called
inflationary theory, and this was a very surprising thing, and it really addressed the number of
important, questions in cosmology. It became a standard paradigm in which early universe is
viewed. We'll talk about this is much more detail later.
Other theories since kept developing better or more ornate versions of inflationary theory. One of
which is the multiverse or megaverse, in which ours is just one of the many inflating bubbles in
some much larger space. But so far that's entirely a hypothetical issue.
1990s, mid-1990s really saw return to the classical cosmology or cosmological tests. Now we
talk about precision cosmology. Two important paths happened there. The first one was studies
of a cosmic microwave background with exquisite precision, both from round, from balloon,
borne instruments, and from space. And cosmic microwave background is not perfectly uniform.
There are some minor fluctuations, like parts in a million, that correspond to density fluctuations
in the early universe. Measuring their size, can constrain cosmological parameters. So this is now
done with great precision, but second thing that happened was use of supernova explosions as
standard candles, with more modern instruments, to push Hubble diagrams much deeper than
was done before, and, it turns out that, that can also constrain cosmological parameters in
important ways, and it became one of the key ingredients in the discovery of dark energy.
So this is just between illustrate the great precision by which we know cosmological parameters.
You can look through them, and there are many different ones. But they're roughly known with
with percent level precision. This is something that earlier cosmologists could only dream about.
So cosmology is now a precision science. And we know properties of the universe with an
amazing accuracy.
Just to jump ahead, here is the punchline. Today, we know that the universe consists of about
73% of so called dark energy. Another 23 percent and so of dark matter which is not made of
regular matter atoms and normal matter which we know about, constitutes only four percent.
This is now determined through many, many different observation using different methods and it
is supported constantly and refined with new measurements. And so generally well accepted
picture, and we'll talk how that's done in more detail later.
So here is essentially cosmic timeline from the modern viewpoint. Universe starts with Big
Bang. There is a brief period of inflation, when universe is less than ten to the minus 32 seconds
old. Then sometime later, physics that we know starts to come into play, such as formation of
first chemical elements, and then slowly formation of of flunctuations, and the formation of
galaxies and there evolution. Obviously the deeper in the past we push in this logorithmic scale,
the less we understand, but it's actually impressive that, we can really address what happened in
the universe when it was say nanoseconds old with reasonably good accuracy.
Here is a slightly different view that would be appealing more to physicists that expresses history
not as a function of time but as a typical energies of particles at that time. Recall that the smaller
and the denser the Universe was, the hotter is was, and kinetic energies were higher. Well today
they're roughly micro background photons, which is energies of one four thousandth of an
electron volt, but early times, the energies are correspondingly larger, and inflation happened
when, particles had energies of ten to the sixteenth giga electron volt. Just for comparison, the
large hadron collider, produces particles of 100 giga electron volts, rest mass energy.
Here we see a simulation of, what the early universe might have looked like, that was produced
by the WMAP space mission team, and it shows fluctuations in the micro background that was
studied with great precision. So at the beginning of one of them, you can sees schematically dark
matter collapsing, making clumps, then stars igniting, first chemical elements are made, galaxies
are slowly assembled.
Another interesting way to visualize the history of the Universe is to say, if all of the age of the
universe. 13.7 billion years today was squeezed in one day. What happened when? Well Big
Bang happens at the beginning. Pretty much everything that predates formation of our solar
system is done in first few hours. On that time scale humans appeared four seconds before
midnight. Four seconds out of an entire 24 hour day. And that gives you a little bit of a scale as
to how significant our history is on cosmic scales.
Martin Rees suggested that cosmology today is now developing in two directions. One of which
he compared to chess, which is theoretical elegant cosmology, which is very precise and clean.
And the other one, which is observations of galaxy formation, which is messy, and he compares
it to mud wrestling. Both of them are important and interesting.