Anonymous asked: What do you think of Lawrence Krauss's "A Universe From Nothing"

The lecture is absolutely wonderful. I’ve watched it several times and I love it each and every single time. The book is also incredible and I highly recommend both of them to anyone interested in the origin and evolution of the Universe.

As a side note, Lawrence Krauss will be in Toronto in the very near future giving a lecture on The Physics of Star Trek. I’m tempted to go even though I’ve never seen anything of Star Trek just because Lawrence Krauss is so badass.

- Jessica

Structure and Evolution of Galaxies
As much as we know about galaxies, the evolution of galaxies is one of the mysteries of the Universe. By looking far out into space and equivalently, far back into time, astronomers can look at the oldest galaxies and make comparisons with modern-day galaxies to get a sense of how they’re evolving. To really understand galaxy evolution, though, one must understand the details of the structure of galaxies; therein lies many wondrous secrets of galaxies. We will focus on spiral galaxies:
The Structure of Spiral Galaxies
Most spiral galaxies have similar features, the most significant of which include a dark matter halo, the galactic disc, spiral arms, a central bulge (most containing a bar-like structure), and a stellar halo.
The dark matter halo is believed to be the main structural component of the Milky Way galaxy whose functional purpose is primarily to hold together the galaxy; without it, the galaxy would not be gravitationally bound. The dark matter halo is situated well beyond the visible galactic disc which is detected using observations like rotation curves of galaxies. Current observations suggest that dark matter accounts for roughly 95% of the Milky Way’s mass, while the other 5% is accounted for with normal, baryonic matter. It is currently believed that dark matter played a significant role in the evolution of galaxies, initiating clumps of matter in the early Universe which eventually lead to structures like galaxies.
The galactic disc is the location of most of the interstellar medium (ISM, composed of gas and dust) and also contains a substantial amount of stars which orbit in roughly circular orbits confined to the galactic plane; it consists of most of the visible matter in the galaxy. The Sun is located about 8.5kpc (kiloparsecs) from the galactic center in the galactic disc. The spiral arms within the galactic disc are the main sites of star formation; the blue spiral arms are indicators of young, hot stars. Within the galactic disc is where most cosmic recycling occurs and as a result, the ISM has a higher metallicity than primordial gas. The metallicity-age relation is important for understanding galaxy evolution. Most open clusters are found in the galactic disc, but some globular clusters (roughly one third) are found here, as well. Open clusters are very young because they are loosely bound and are easily disrupted. Open clusters have high metallicities because they are formed from more recently formed ISM which has had time to become enriched with metals. In contrast, globular clusters are very old as they are strongly gravitationally bound. Since globular clusters consist of the oldest stars in the galaxy, the metallicity-age relation suggests that they should have very low metallicities but those found in the galactic disc have unusually high metallicities; this suggests that cosmic recycling enriches the contents of the globular clusters.  
The stellar halo extends further than the galactic disc itself and is roughly spherical in shape. Here lie roughly two thirds of the galaxy’s globular clusters. The globular clusters found here, as expected, have very low metallicities because they are young and do not get contaminated by the cosmic recycling that takes place in the galactic disc. The stellar halo’s star formation has long since ceased; there is very little ISM left in the stellar halo. The stars within the stellar halo have largely elliptical and highly irregular orbital motions which are not confined to the galactic plane.
What Does Any of This Tell Us About Galaxy Evolution?
Without the dark matter halo, our galaxy wouldn’t be gravitationally bound the way it is today. With this, along with the fact that dark matter consists of 95% of the galaxy’s mass, it is suggested that dark matter played a key role in the initial formation of galaxies. The spiral arms of the galaxy suggest that the galaxy is rotating, which is well supported by observations. Older stars contained in the stellar halo having irregular, elliptical orbits and younger stars confined in the galactic disc having regular, circularized orbits suggests that the galaxy started out in a disordered state which over time settled down to a more orderly state; the galaxy originated as a large gas cloud that eventually collapsed and through the conservation of angular momentum, became a flat, rotating disc. The fact that globular clusters within the galactic disc have unusually high metallicities suggest that cosmic recycling is the origin; as time progresses, high mass stars explode and contaminate the surrounding ISM to further enrich future stars’ chemical composition. Furthermore, globular clusters which were once believed to originate from the Milky Way are now believed to having been captured by the Sagittarius Dwarf galaxy, which suggests that even though the galactic halo is the oldest component of the galaxy, it is still a dynamic part of the Milky Way. Currently, the Milky Way is in a collision course with its neighbour, the Andromeda galaxy. In about 4 billion years, the two spiral galaxies will collide to form what astronomers like to call the Milkomeda galaxy.

Structure and Evolution of Galaxies

As much as we know about galaxies, the evolution of galaxies is one of the mysteries of the Universe. By looking far out into space and equivalently, far back into time, astronomers can look at the oldest galaxies and make comparisons with modern-day galaxies to get a sense of how they’re evolving. To really understand galaxy evolution, though, one must understand the details of the structure of galaxies; therein lies many wondrous secrets of galaxies. We will focus on spiral galaxies:

The Structure of Spiral Galaxies

Most spiral galaxies have similar features, the most significant of which include a dark matter halo, the galactic disc, spiral arms, a central bulge (most containing a bar-like structure), and a stellar halo.

The dark matter halo is believed to be the main structural component of the Milky Way galaxy whose functional purpose is primarily to hold together the galaxy; without it, the galaxy would not be gravitationally bound. The dark matter halo is situated well beyond the visible galactic disc which is detected using observations like rotation curves of galaxies. Current observations suggest that dark matter accounts for roughly 95% of the Milky Way’s mass, while the other 5% is accounted for with normal, baryonic matter. It is currently believed that dark matter played a significant role in the evolution of galaxies, initiating clumps of matter in the early Universe which eventually lead to structures like galaxies.

The galactic disc is the location of most of the interstellar medium (ISM, composed of gas and dust) and also contains a substantial amount of stars which orbit in roughly circular orbits confined to the galactic plane; it consists of most of the visible matter in the galaxy. The Sun is located about 8.5kpc (kiloparsecs) from the galactic center in the galactic disc. The spiral arms within the galactic disc are the main sites of star formation; the blue spiral arms are indicators of young, hot stars. Within the galactic disc is where most cosmic recycling occurs and as a result, the ISM has a higher metallicity than primordial gas. The metallicity-age relation is important for understanding galaxy evolution. Most open clusters are found in the galactic disc, but some globular clusters (roughly one third) are found here, as well. Open clusters are very young because they are loosely bound and are easily disrupted. Open clusters have high metallicities because they are formed from more recently formed ISM which has had time to become enriched with metals. In contrast, globular clusters are very old as they are strongly gravitationally bound. Since globular clusters consist of the oldest stars in the galaxy, the metallicity-age relation suggests that they should have very low metallicities but those found in the galactic disc have unusually high metallicities; this suggests that cosmic recycling enriches the contents of the globular clusters. 

The stellar halo extends further than the galactic disc itself and is roughly spherical in shape. Here lie roughly two thirds of the galaxy’s globular clusters. The globular clusters found here, as expected, have very low metallicities because they are young and do not get contaminated by the cosmic recycling that takes place in the galactic disc. The stellar halo’s star formation has long since ceased; there is very little ISM left in the stellar halo. The stars within the stellar halo have largely elliptical and highly irregular orbital motions which are not confined to the galactic plane.

What Does Any of This Tell Us About Galaxy Evolution?

Without the dark matter halo, our galaxy wouldn’t be gravitationally bound the way it is today. With this, along with the fact that dark matter consists of 95% of the galaxy’s mass, it is suggested that dark matter played a key role in the initial formation of galaxies. The spiral arms of the galaxy suggest that the galaxy is rotating, which is well supported by observations. Older stars contained in the stellar halo having irregular, elliptical orbits and younger stars confined in the galactic disc having regular, circularized orbits suggests that the galaxy started out in a disordered state which over time settled down to a more orderly state; the galaxy originated as a large gas cloud that eventually collapsed and through the conservation of angular momentum, became a flat, rotating disc. The fact that globular clusters within the galactic disc have unusually high metallicities suggest that cosmic recycling is the origin; as time progresses, high mass stars explode and contaminate the surrounding ISM to further enrich future stars’ chemical composition. Furthermore, globular clusters which were once believed to originate from the Milky Way are now believed to having been captured by the Sagittarius Dwarf galaxy, which suggests that even though the galactic halo is the oldest component of the galaxy, it is still a dynamic part of the Milky Way. Currently, the Milky Way is in a collision course with its neighbour, the Andromeda galaxy. In about 4 billion years, the two spiral galaxies will collide to form what astronomers like to call the Milkomeda galaxy.

Anonymous asked: What is the Cosmos?

The term “universe” was traditionally used to encompass everything that exists. If if exists, it is part of the universe. Now, there are multiverse theories which suggest that our Universe isn’t the only universe. It has now become widely popular to refer to “everything” as the Cosmos and our particular universe the Universe. It seems like silly semantics, but we no longer discuss only our Universe because many people believe there exists something outside our Universe. Cosmos is the next best thing; it encompasses all universes. Our Universe is part of the Cosmos.

- Jessica

Anonymous asked: can you look through a strong telescope back into time ?

Looking out into space is looking back into time; that’s the beauty of it. You don’t need a big telescope to do so. In fact, you don’t even need a telescope at all to look back into time.

Consider the fact that light travels at a finite speed. The Sun is 1 AU (one astronomical unit) away from the Earth. 1 AU = 150 million km. It takes light leaving the surface of the Sun approximately 8 minutes to reach the Earth. This means that if you were to look at the Sun (which you should never do), what you’re actually seeing is 8 minutes into the past! Seeing something like the Andromeda galaxy is looking even farther back into time because it is even further away from us. The Andromeda galaxy (our neighbour), is 2.5 million light years away from us, which means that it takes light 2.5 million years to get to us and thus when you look at Andromeda, you are actually looking 2.5 million years into the past. The further out into space you look, the further back in time you are seeing. This is simply because the light you are seeing travels at a finite speed and by the time the light reaches us, time has passed since the photons left. The furthest back in time we can see is looking at the CMB radiation which was released when the Universe was only 400,000 years old. This light is detected as radio waves and sadly, we cannot see any further back into time (for the same reason we can’t see inside the Sun.) Otherwise, in principle, we would be able to see the beginning of the Universe! Can you imagine?!

-Jessica

Anonymous asked: Why does the expansion of the universe increase the wavelength of photons, but not their size?

Simply because photons don’t have a physical size! It’s strange, isn’t it? Photons have linear and angular (spin) momentum but they don’t even have a physical size. They are dimensionless particles. Similarly with electrons. The characteristic of such particles that does have a size, though, is their wavelength which does get stretched.

The smallest structures that are affected by the expansion rate of the Universe are superclusters of galaxies. Our own supercluster (the Virgo supercluster) is marginally bound by gravitational attraction. Everything within our supercluster of galaxies is being held together by gravitational attraction against the expansion of the Universe.

- Jessica

Anonymous asked: In your post "post/21800557534/if-the-universe-is-expanding-does-that-mean-that" you stated that "Astronomers commonly probe the depths of space that reveal redshifts larger than one (z>1) which suggests that these observed objects are receding at greater speeds than that of light." If, for example, distant stars are receding faster than the speed of light, then how do the photons they emit reach us? Or are astronomers probing the depths of space without the aid of photons?

Very good question! This startled me at first too, and I’m glad you picked up on this. It was not a mistake; for example, the Cosmic Microwave Background radiation had a temperature of approximately 3,000K when it was emitted and measurements suggest it has a current temperature of ~2.725K. For simplicity, let’s say the current CMB temperature is 3K. The CMB, then, had been stretched by a factor of 1,000 and thus had been redshifted by about 1,000. To merely observe the CMB radiation which happened when the universe was about 400,000 years old is to probe so far into space that the redshift is as high as 1,000.

Now, this is where your confusion arises; does this mean that objects as far as the CMB are moving 1,000 times the speed of light? Is the CMB that we detect moving at 1,000 times the speed of light? Well, no. There’s two different types of redshifts that can be measured: the real motion of objects (which is given by the Doppler Shift of the motion of objects in the radial direction which we can measure from their emission spectra) and then there’s measuring the expansion rate of the Universe itself. At low redshifts, the measurements are significantly due to the real motion of objects, but at high enough redshifts, especially redshifts as high as 1,000, that is certainly a measurement of the expansion rate of the Universe itself.

Though objects such as galaxies cannot move faster than light, the expansion rate of the Universe is an exception. In my first year of my undergraduate studies, what I’m about to tell you was by far the coolest thing I learned: on larger scales, nothing is actually moving. Distant galaxies and their spectra suggest that they are moving away from us (that’s what their redshifts tell us), but in reality, it’s actually the expansion of the Universe that is causing their redshifts. It is the space BETWEEN GALAXIES that is expanding, and while photons are traveling within this space, these photons get stretched by the expansion of the Universe itself which makes them look red. Thus, though we talk about recessional velocities of distant galaxies, it’s actually the expansion rate of the Universe that we’re measuring, which is allowed to exceed the speed of light. So, the CMB isn’t receding at 1,000 times the speed of light… the Universe at that point is expanding at 1,000 times the speed of light.

Thank you for this wonderful question. I always love explaining this to people.

- Jessica

Tides
Contrary to what one may think, tides are caused by a differential force, not simply the mere force of gravity. Consider the fact that there are two tidal bulges on Earth; if tides were simply caused by the gravitational pull of our Moon, there would only be one tidal bulge on the side of Earth closest to the Moon when in fact, there is one tidal bulge on each side. So how can we explain this? Let’s go outside of our comfort zones and into the non-inertial reference frame of the rotating Earth.
In a non-inertial reference frame, we need to take into consideration what physicists call “fictitious” or “inertial” forces. (Now, before we go any further, physicists and astronomers certainly call it like it is, but fictitious forces are just as real as “real” forces. You swerve around a corner and you get pressed up against the inside of the car door or your friend – there’s no denying the reality of these forces.) Such inertial forces include the centrifugal force (not to be confused with the centripetal force) and the coriolis force. In the rotating reference frame of the Earth, we only need to keep in mind the centrifugal force – the apparent outward force caused by our rotating reference frame attached to the Earth – and the gravitational force of the Moon on the Earth. Assuming a constant rate of rotation, the centrifugal force on all points of the Earth will be the same, but due to the varying distances from the Moon, the gravitational force will vary at different points on the Earth.
This varying gravitational force along with the centrifugal force is what causes the tides. At the point on the earth closest to the Moon, the gravitational force is strongest here whereas at the point farthest from the Moon, the gravitational force is weakest. Where the gravitational force is the strongest (near the Moon), the tidal bulge is caused by an excess of the gravitational force. Where the gravitational force is the weakest (farthest from the Moon), the tidal bulge is caused by the fact that the centrifugal force exceeds the strength of the gravitational force caused by the Moon, which is why the tidal bulge opposite the side of the Moon points in the opposite direction.  
Effects
Within the tidal bulges of the Earth-Moon system, friction is causing the system to lose energy. The rotation of the Earth is causing friction between Earth and the tidal bulges and since energy is being lost by this process, the rotation of the Earth is being slowed down. While energy of the system as a whole is not conserved, angular momentum is. The result? Angular momentum is being transferred from the Earth to the Moon, which is causing a net orbital separation between the Earth and the Moon. While the Moon is slowing leaving us, there will come a time where the Moon will become aligned with both of the Earth’s tidal bulges, where the Earth will stop losing energy via friction, and the Moon will be in synchronous rotation with the Earth. Astrophysicists suggests that this will occur when the length of the day is about 47 current Earth days long.

Tides

Contrary to what one may think, tides are caused by a differential force, not simply the mere force of gravity. Consider the fact that there are two tidal bulges on Earth; if tides were simply caused by the gravitational pull of our Moon, there would only be one tidal bulge on the side of Earth closest to the Moon when in fact, there is one tidal bulge on each side. So how can we explain this? Let’s go outside of our comfort zones and into the non-inertial reference frame of the rotating Earth.

In a non-inertial reference frame, we need to take into consideration what physicists call “fictitious” or “inertial” forces. (Now, before we go any further, physicists and astronomers certainly call it like it is, but fictitious forces are just as real as “real” forces. You swerve around a corner and you get pressed up against the inside of the car door or your friend – there’s no denying the reality of these forces.) Such inertial forces include the centrifugal force (not to be confused with the centripetal force) and the coriolis force. In the rotating reference frame of the Earth, we only need to keep in mind the centrifugal force – the apparent outward force caused by our rotating reference frame attached to the Earth – and the gravitational force of the Moon on the Earth. Assuming a constant rate of rotation, the centrifugal force on all points of the Earth will be the same, but due to the varying distances from the Moon, the gravitational force will vary at different points on the Earth.

This varying gravitational force along with the centrifugal force is what causes the tides. At the point on the earth closest to the Moon, the gravitational force is strongest here whereas at the point farthest from the Moon, the gravitational force is weakest. Where the gravitational force is the strongest (near the Moon), the tidal bulge is caused by an excess of the gravitational force. Where the gravitational force is the weakest (farthest from the Moon), the tidal bulge is caused by the fact that the centrifugal force exceeds the strength of the gravitational force caused by the Moon, which is why the tidal bulge opposite the side of the Moon points in the opposite direction. 

Effects

Within the tidal bulges of the Earth-Moon system, friction is causing the system to lose energy. The rotation of the Earth is causing friction between Earth and the tidal bulges and since energy is being lost by this process, the rotation of the Earth is being slowed down. While energy of the system as a whole is not conserved, angular momentum is. The result? Angular momentum is being transferred from the Earth to the Moon, which is causing a net orbital separation between the Earth and the Moon. While the Moon is slowing leaving us, there will come a time where the Moon will become aligned with both of the Earth’s tidal bulges, where the Earth will stop losing energy via friction, and the Moon will be in synchronous rotation with the Earth. Astrophysicists suggests that this will occur when the length of the day is about 47 current Earth days long.

Spacetime
In 1905 Einstein showed that space and time are two parts of a unity: spacetime. In our ordinary life, however, we treat space and time differently, measuring one in meters and the other in seconds. They look so distinct in our experience that it seems obvious to measure them in different ways.
If, however, we accept the Einstein’s conclusion about spacetime, we must deal with space and time on an equal footing. So let’s do it!
How much space is 1 hour of space? What about 2 meters of time? Even if hard to grasp in the beginning, these questions are completely reasonable. There is, in fact, a conversion factor between space and time that physicists use to call c and it’s approximately equal to 299,792,458 meters per second.
Wait a minute, is that the speed of light?
Yes. Actually, there’s no particular reason to call it the speed of light, it may be called the speed of graviton as well. As a matter of fact, it’s the speed at which travels every massless particle.
Keeping in mind this new idea we can see that 1 hour of space is the space that light travels in 1 hour (the space traveled by light in one year is probably more familiar to us and we call it one light-year). Similarly 2 meters of time is the time that takes light to travel 2 meters.
This however doesn’t resolve our whole problem, since we’re still left with human-invented units like seconds and meters upon which the universe surely can’t be founded on. To create a description that works independently of the units we must, finally, use the same units for space and time. In this way we have that light travels at 1 meter per meter (that is, each meter of time light travels 1 meter of space), or 1 second per second, or 1 inch per inch etc. In this description the units cancels out and the factor c, the speed of light, has simply value 1. It becomes evident, then, as just a unitless factor of conversion between space and time.
Sources:
Spacetime Physics by J.A. Wheeler & E.F. Taylor
http://en.wikipedia.org/wiki/Spacetime

Spacetime

In 1905 Einstein showed that space and time are two parts of a unity: spacetime. In our ordinary life, however, we treat space and time differently, measuring one in meters and the other in seconds. They look so distinct in our experience that it seems obvious to measure them in different ways.

If, however, we accept the Einstein’s conclusion about spacetime, we must deal with space and time on an equal footing. So let’s do it!

How much space is 1 hour of space? What about 2 meters of time? Even if hard to grasp in the beginning, these questions are completely reasonable. There is, in fact, a conversion factor between space and time that physicists use to call c and it’s approximately equal to 299,792,458 meters per second.

Wait a minute, is that the speed of light?

Yes. Actually, there’s no particular reason to call it the speed of light, it may be called the speed of graviton as well. As a matter of fact, it’s the speed at which travels every massless particle.

Keeping in mind this new idea we can see that 1 hour of space is the space that light travels in 1 hour (the space traveled by light in one year is probably more familiar to us and we call it one light-year). Similarly 2 meters of time is the time that takes light to travel 2 meters.

This however doesn’t resolve our whole problem, since we’re still left with human-invented units like seconds and meters upon which the universe surely can’t be founded on. To create a description that works independently of the units we must, finally, use the same units for space and time. In this way we have that light travels at 1 meter per meter (that is, each meter of time light travels 1 meter of space), or 1 second per second, or 1 inch per inch etc. In this description the units cancels out and the factor c, the speed of light, has simply value 1. It becomes evident, then, as just a unitless factor of conversion between space and time.

Sources:

The Big Bang
The Big Bang is a beautiful theory which is an effort to understand where the universe came from. Some of the most fundamental questions concerning our origins, such as that of the elements, can be explained with the Big Bang theory. But just where did everything come from? What existed before the Big Bang? Where did space come from? And what caused the Big Bang? Well, the simple answer is: We don’t know.
We know the universe is expanding; it’s accelerating, actually. This means that yesterday, the universe was a little bit smaller than it is today. A month ago, it was even smaller. A year ago, smaller still. Turning the clock backwards, the universe seems to be getting smaller, the galaxies closer together. If we go further enough back in time, the universe was so small that everything was contained in a point of space and time. Everything that exists today; you, me, the Earth, our Galaxy, everything came from this point.
Approximately 14.6 billion years ago, the Universe was created and it was very hot. Radiation (simply photons) dominated the early universe which cooled down as it expanded. Analysis of the CMB data suggests that the universe is a perfect blackbody; a higher blackbody temperature means typical photons have higher energies. In the early universe, these photons were so energetic that they produced matter-antimatter particles copiously seemingly out of “nothing” which can be explained using Einstein’s E=mc² formula (see this post.) The early universe was constantly creating matter and antimatter which quickly annihilated; this is the Particle Era. The universe was bubbling with matter, the prerequisite for everything in our Universe. Since our Universe is made of matter, and not antimatter, a baryonic asymmetry is proposed to be the origin of our matter dominated Universe.
Once the mean photon energy drops below ~1MeV, nuclei may be formed. This is the nuclear binding energy and thus, the Nucleosynthesis Era. During the Nucleosynthesis Era, the universe is one big nuclear reactor. This era sets the primordial chemical composition of the universe: 76% Hydrogen and 24% Helium.
The Nucleosynthesis Era is followed by the Era of Nuclei. Photon energies are at this point beyond the electron binding energy (~1eV). This era of the universe is foggy since photons are continuously being scattered by nuclei. At the very special moment during which photon energies drop below the electron binding energy, electrons may then bind to nuclei to form the first atoms - the fog is lifted. The Universe, during the era of atoms, becomes transparent. Photons are no longer being continuously scattered and they are suddenly released. This release of photons during the Era of Atoms is the origin of the Cosmic Microwave Background and is a significant use of study. Recall that beyond the CMB, before stable atoms are made, the universe is still foggy. It is for this reason that we cannot see beyond this point in the universe.
Not only can we not see past this point in the universe, but we cannot (yet) study what is happening at the moment of the Big Bang. There are no mathematical tools that can be used at the moment of the Big Bang, and thus, we cannot study what happened before the Big Bang. The current laws of physics seem to break down at the singularity in the beginning of the Universe, similar to what happens when we attempt to understand what happens inside a black hole. What caused the Big Bang is still a mystery, and there is still a lot left to discover, but we have achieved a lot in our understanding. The origin of our species, of the stars in the sky, of the elements that compose our Universe, can all be explained with this elegant theory.

The Big Bang

The Big Bang is a beautiful theory which is an effort to understand where the universe came from. Some of the most fundamental questions concerning our origins, such as that of the elements, can be explained with the Big Bang theory. But just where did everything come from? What existed before the Big Bang? Where did space come from? And what caused the Big Bang? Well, the simple answer is: We don’t know.

We know the universe is expanding; it’s accelerating, actually. This means that yesterday, the universe was a little bit smaller than it is today. A month ago, it was even smaller. A year ago, smaller still. Turning the clock backwards, the universe seems to be getting smaller, the galaxies closer together. If we go further enough back in time, the universe was so small that everything was contained in a point of space and time. Everything that exists today; you, me, the Earth, our Galaxy, everything came from this point.

Approximately 14.6 billion years ago, the Universe was created and it was very hot. Radiation (simply photons) dominated the early universe which cooled down as it expanded. Analysis of the CMB data suggests that the universe is a perfect blackbody; a higher blackbody temperature means typical photons have higher energies. In the early universe, these photons were so energetic that they produced matter-antimatter particles copiously seemingly out of “nothing” which can be explained using Einstein’s E=mc² formula (see this post.) The early universe was constantly creating matter and antimatter which quickly annihilated; this is the Particle Era. The universe was bubbling with matter, the prerequisite for everything in our Universe. Since our Universe is made of matter, and not antimatter, a baryonic asymmetry is proposed to be the origin of our matter dominated Universe.

Once the mean photon energy drops below ~1MeV, nuclei may be formed. This is the nuclear binding energy and thus, the Nucleosynthesis Era. During the Nucleosynthesis Era, the universe is one big nuclear reactor. This era sets the primordial chemical composition of the universe: 76% Hydrogen and 24% Helium.

The Nucleosynthesis Era is followed by the Era of Nuclei. Photon energies are at this point beyond the electron binding energy (~1eV). This era of the universe is foggy since photons are continuously being scattered by nuclei. At the very special moment during which photon energies drop below the electron binding energy, electrons may then bind to nuclei to form the first atoms - the fog is lifted. The Universe, during the era of atoms, becomes transparent. Photons are no longer being continuously scattered and they are suddenly released. This release of photons during the Era of Atoms is the origin of the Cosmic Microwave Background and is a significant use of study. Recall that beyond the CMB, before stable atoms are made, the universe is still foggy. It is for this reason that we cannot see beyond this point in the universe.

Not only can we not see past this point in the universe, but we cannot (yet) study what is happening at the moment of the Big Bang. There are no mathematical tools that can be used at the moment of the Big Bang, and thus, we cannot study what happened before the Big Bang. The current laws of physics seem to break down at the singularity in the beginning of the Universe, similar to what happens when we attempt to understand what happens inside a black hole. What caused the Big Bang is still a mystery, and there is still a lot left to discover, but we have achieved a lot in our understanding. The origin of our species, of the stars in the sky, of the elements that compose our Universe, can all be explained with this elegant theory.

returntothestars:

So, I wonder, does this observation completely rule out the “open” and “closed” models, or might it just mean that the universe is so big, that its curvature is not noticeable to us? Just as an ant on a beach ball might think itself on a flat surface.

An explanation from someone with a firmer grasp on such things than I have would be much aprreactiated.

With more observational data than this, the open and closed topologies of the universe have been ruled out but of course, we only have supporting data of a flat universe and we cannot say with certainty that the universe is flat, but rather that all evidence suggests that it is. For example, data from distant supernovae are correlated with a flat universe model. WMAP data from calculating the density of the universe completely unrelated to the CMB data suggests that the density of the universe is almost exactly that of the critical density. Since the density parameter of the universe is a ratio of the actual density of the universe to the critical density which would make the universe flat, to say that the density parameter is almost exactly equal to 1 (ie: almost exactly at unity,) we can conclude that the universe is almost perfectly flat. Furthermore, as the universe expands, the density of the universe changes while the critical density of the universe changes. This means that the universe will always remain flat (despite dark energy, the universe will not suddenly become open, for example.) The CMB data seems to be the easiest way to visualize and understand why the universe is flat.

Jessica.