Definition and Basics:
Key Concepts
As cosmologists, the main paradigms we work under are known (rather inappropriately as the big-bang model for the global evolution of the universe and the gravitational instability paradigm for the formation of objects or structure in the universe.
The big bang model says that the universe began hot and dense but is expanding and cooling.
The gravitational instability models says: we know large masses like the earth attract, for example by the fact that you remain on the surface of the earth rather than flung off of it as it spins, like water off a wet dog. Even a small mass attracts, so that small ripples in the mass density early on in the history of the universe can grow into the galaxies we see today in the night sky.
- Big Bang: the universe began hot and dense and thereafter expanded and cooled.
- Gravitational Instability: Stars, galaxies, clusters of galaxies ("structure") formed from the gravitational pull of small density ripples in the early universe
As cosmologists, the main paradigms we work under are known (rather inappropriately as the big-bang model for the global evolution of the universe and the gravitational instability paradigm for the formation of objects or structure in the universe.
The big bang model says that the universe began hot and dense but is expanding and cooling.
The gravitational instability models says: we know large masses like the earth attract, for example by the fact that you remain on the surface of the earth rather than flung off of it as it spins, like water off a wet dog. Even a small mass attracts, so that small ripples in the mass density early on in the history of the universe can grow into the galaxies we see today in the night sky.
History-
“When we look out in the sky, we're actually looking backwards in time. Light from more distant objects take longer to reach us and thus we are observing now how they appeared in the past. We can see back a few billion years with the light of galaxies. The microwave light of the background shines from long ago in an infant universe 300,000 years old (the epoch of "last scattering") and illuminates the particle soup that existed before this time. This soup has a very smooth consistency and is composed of fundamental particles like electrons, protons, helium nuclei, neutrinos”.
Introduction and Cosmic Expansion-
The big bang finds strong evidence from the recession of the galaxies, or when
we look out in the night sky, distant objects appear to be flying away from us. Also cosmic microwave background (the fact that the universe is bathed in a primordial light that shares evidence of the hotter and denser big bang). This also supports the gravitational instability paradigm (the picture that gravity can form wrinkles or overlaps with in space and time. There is a relation between the recession and expansion. “Imagine you're standing at the north pole and think boy, I'd much rather be at the basking the sun at some equatorial paradise. You look up the distance to it and plan your trip. Right before you leave, you think, I'd better check the distance again. Unbeknownst to you the radius of the earth has expanded in the meantime. To your surprise the distance to your equatorial paradise is now larger. You think, that's funny, paradise seems to be receding from me!” This is what we observe with the distant galaxies. As a result of space itself expanding it looks as if the galaxies around us are all moving away. The wavelength of light also stretches with the expansion so that visible light a millionths of meters wavelengths gets stretched into microwaves.”Given that light behaves this way, it's easy to see how the sea of primordial light we observe in microwaves supports the hot expanding (big bang) model. Run these pictures backwards in time. The wavelength of light becomes shorter and shorter and matter becomes denser and denser implying that the universe began in a hot dense state”. |
This image shoes the basic ideas of an expanding universe and its effects on waves. We see that wave lengths expand as the universe moves outward and the distance between two point spreads. This is what we have observed from earth and is also why we can't see all primordial light (it is still traveling towards our eyes and it has deformed greatly over time). Image was provided by the Chicago State University.
http://background.uchicago.edu/~whu/beginners/expansion.html |
Expanding at and Accelerated Rate-
Hubble's law- This is an observable theory used in regard to Cosmic Expansion and serves as one of the most reliable pieces of evidence for the Big Bang.
This concept was contributed by Edwin Hubble, however, was first suggested in the general relativity equations by Georges Lemaitre in a 1927 article which he proposed the expansion of the universe (cosmic expansion) and suggested a rate at which the expansion increase (now known as Hubble's Constant). Two years later Edwin Hubble confirmed the existence of the cosmic expansion Hubble inferred the recession velocity of the objects for their redshifts.
This is often expressed by the equation v = H0D, with H0 the Hubble constant between the "proper distance" D to a galaxy, and its velocity v (i.e. the derivative of proper distance with respect to cosmological time coordinate. This is giving the speed in km/s in which the galaxy 1 megaparsec away is accelerating at.
“The accelerating expansion of the universe is the observation that the universe appears to be expanding at an increasing rate. In formal terms, this means that the cosmic scale factor at has a positive second derivative, so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time
The expansion of the universe has been accelerating since the universe entered its dark-energy-dominated era, at redshift z≈0.4 (roughly 5 billion years ago). Within the framework of general relativity, an accelerating expansion can be accounted for by a positive value of the cosmological constant Λ, equivalent to the presence of a positive vacuum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positive Λ) is used in the current standard model of cosmology, known as ΛCDM ("Lambda cold dark matter").
The accelerated expansion was discovered in 1998, when two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team simultaneously obtained results suggesting an acceleration in the expansion of the universe by using distant type Ia supernovae as standard candles. The discovery was unexpected, cosmologists at the time expecting a deceleration in the expansion of the universe, and amounts to the realization that the universe is currently in a "dark-energy-dominated era”.
This concept was contributed by Edwin Hubble, however, was first suggested in the general relativity equations by Georges Lemaitre in a 1927 article which he proposed the expansion of the universe (cosmic expansion) and suggested a rate at which the expansion increase (now known as Hubble's Constant). Two years later Edwin Hubble confirmed the existence of the cosmic expansion Hubble inferred the recession velocity of the objects for their redshifts.
This is often expressed by the equation v = H0D, with H0 the Hubble constant between the "proper distance" D to a galaxy, and its velocity v (i.e. the derivative of proper distance with respect to cosmological time coordinate. This is giving the speed in km/s in which the galaxy 1 megaparsec away is accelerating at.
“The accelerating expansion of the universe is the observation that the universe appears to be expanding at an increasing rate. In formal terms, this means that the cosmic scale factor at has a positive second derivative, so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time
The expansion of the universe has been accelerating since the universe entered its dark-energy-dominated era, at redshift z≈0.4 (roughly 5 billion years ago). Within the framework of general relativity, an accelerating expansion can be accounted for by a positive value of the cosmological constant Λ, equivalent to the presence of a positive vacuum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positive Λ) is used in the current standard model of cosmology, known as ΛCDM ("Lambda cold dark matter").
The accelerated expansion was discovered in 1998, when two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team simultaneously obtained results suggesting an acceleration in the expansion of the universe by using distant type Ia supernovae as standard candles. The discovery was unexpected, cosmologists at the time expecting a deceleration in the expansion of the universe, and amounts to the realization that the universe is currently in a "dark-energy-dominated era”.
Metric Expansion of Space-
The metric expansion of space is the increase of the distance between two parts of the universe over time.
“It is an intrinsic expansion whereby the scale of space itself changes. This is different from other examples of expansions and explosions in that, as far as observations can ascertain, it is a property of the entirety of the universe rather than a phenomenon that can be contained and observed from the outside.
Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the FLRW metric and is a generic property of the universe. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such things do not expand at the metric expansion rate as the universe ages. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the universe given the matter density and average expansion rate.
At the end of the early universe's inflationary period, all the matter and energy in the universe was set on an inertial trajectory consistent with the equivalence principle and Einstein's general theory of relativity and this is when the precise and regular form of the universe's expansion had its origin (that is, matter in the universe is separating because it was separating in the past due to the inflaton field).
According to measurements, the universe's expansion rate was decelerating until about 5 billion years ago due to the gravitational attraction of the matter content of the universe, after which time the expansion began accelerating. The source of this acceleration is currently unknown. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models as a way to explain the acceleration. According to the simplest extrapolation of the currently-favored cosmological model (known as "ΛCDM"), this acceleration becomes more dominant into the future.
While special relativity prohibits objects from moving faster than light with respect to a local reference frame where spacetime can be treated as flat and unchanging, it does not apply to situations where spacetime curvature or evolution in time become important. These situations are described by general relativity, which allows the separation between two distant objects to increase faster than the speed of light, although the definition of "distance" here is somewhat different that used in an inertial frame. The definition of distance used here is the summation or integration of local comoving distances, all done at constant local proper time. For example, galaxies that are more than the Hubble radius, approximately 4.5 gigaparsecs or 14.7 billion light-years, away from us have a recession speed that is faster than the speed of light. Visibility of these objects depends on the exact expansion history of the universe. Light that is emitted today from galaxies beyond the cosmological event horizon, about 5 gigaparsecs or 16 billion light-years, will never reach us, although we can still see the light that these galaxies emitted in the past.
Because of the high rate of expansion, it is also possible for a distance between two objects to be greater than the value calculated by multiplying the speed of light by the age of the universe. These details are a frequent source of confusion among amateurs and even professional physicists”.
“It is an intrinsic expansion whereby the scale of space itself changes. This is different from other examples of expansions and explosions in that, as far as observations can ascertain, it is a property of the entirety of the universe rather than a phenomenon that can be contained and observed from the outside.
Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the FLRW metric and is a generic property of the universe. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such things do not expand at the metric expansion rate as the universe ages. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the universe given the matter density and average expansion rate.
At the end of the early universe's inflationary period, all the matter and energy in the universe was set on an inertial trajectory consistent with the equivalence principle and Einstein's general theory of relativity and this is when the precise and regular form of the universe's expansion had its origin (that is, matter in the universe is separating because it was separating in the past due to the inflaton field).
According to measurements, the universe's expansion rate was decelerating until about 5 billion years ago due to the gravitational attraction of the matter content of the universe, after which time the expansion began accelerating. The source of this acceleration is currently unknown. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models as a way to explain the acceleration. According to the simplest extrapolation of the currently-favored cosmological model (known as "ΛCDM"), this acceleration becomes more dominant into the future.
While special relativity prohibits objects from moving faster than light with respect to a local reference frame where spacetime can be treated as flat and unchanging, it does not apply to situations where spacetime curvature or evolution in time become important. These situations are described by general relativity, which allows the separation between two distant objects to increase faster than the speed of light, although the definition of "distance" here is somewhat different that used in an inertial frame. The definition of distance used here is the summation or integration of local comoving distances, all done at constant local proper time. For example, galaxies that are more than the Hubble radius, approximately 4.5 gigaparsecs or 14.7 billion light-years, away from us have a recession speed that is faster than the speed of light. Visibility of these objects depends on the exact expansion history of the universe. Light that is emitted today from galaxies beyond the cosmological event horizon, about 5 gigaparsecs or 16 billion light-years, will never reach us, although we can still see the light that these galaxies emitted in the past.
Because of the high rate of expansion, it is also possible for a distance between two objects to be greater than the value calculated by multiplying the speed of light by the age of the universe. These details are a frequent source of confusion among amateurs and even professional physicists”.
Principles of the Start of the Universe-
The universal theory is the big bang which is a model for cosmic evolution and the gravitational instability paradigm regarding the formation of objects. The big bang model says that the universe began as a hot and dense ball that is expanding and cooling
There is also the gravitational instability models which state, “we know larger masses like earth attract, for example by the fact that you remain on the surface of the earth rather than flung off of it as it spins, like water off a wet dog. Even a small mass attracts, so that small ripples in the mass density early on in the history of the universe can grow into the galaxies we see today in the night sky”.
Because gravitational instability creates hills and valleys in time and space the matter will tend to fall into the valleys and that’s where the galaxies and celestial masses will form.
There is also the gravitational instability models which state, “we know larger masses like earth attract, for example by the fact that you remain on the surface of the earth rather than flung off of it as it spins, like water off a wet dog. Even a small mass attracts, so that small ripples in the mass density early on in the history of the universe can grow into the galaxies we see today in the night sky”.
Because gravitational instability creates hills and valleys in time and space the matter will tend to fall into the valleys and that’s where the galaxies and celestial masses will form.
Properties of Cosmic Light-
![Picture](/uploads/5/9/8/2/59827639/em-spectrum-properties-edit_1.png?399)
Due to the cosmic expansion, the microwave background is cooling (about 3 degrees above zero now). This causes its wavelength to be streched out of the visible section into the much slower microwave regime. In relation to photons (the particles of life) there are a large amount of theme in the microwave background (approximately 400 per cubic centimeter). These travel very vastly throughout the cosmos. Because the human eye can not observe these microwaves scientists have turned to experimental physics. “They use specially tuned antennas and receivers to detect microwave photons. When they look out on the sky, what they see is remarkably uniform across the sky. To 1 part in 100,000 the temperature in one direction is the same as in the other. However, there are small variations (or ripples) in the temperature that we observe. These wrinkles are useful in xx determining how objects like galaxies formed in the universe”.
Spatial Curvature-
Curvature of space is the same as gravity and is a measure of the amount of matter presented in the universe. This curvature is measured by its lensing effect on spots of the microwave background. If the universe has a large uniform curve it is massive enough to recollapse ending the universe.
It has been shown that microwave background radiation can be used to find the origins of structures in the universe. The M.B. can also help to understand the universe's fate. Einstein told us that matter curves space, the more the matter the more space is curved.
“Say my friend and I were in our tropical paradise near the equator. Paradise is boring he says, let's go north. We start out 100 feet apart and walk due north. Euclid told us that so long as we walk in perfectly straight lines, we should never meet, yet we all know that my friend and I will meet at the north pole. In my ignorance, I might say that there is a strange attractive force between my friend and I and label that "gravity". But in truth what happened was the surface of the earth curved beneath us as we walked”.
Using this example and similar examples, Einstein came to a revelation that gravity is no more than the curvature of spacetime. So curvature is created by matter itself.
It is unknown if the universe is a sphere, we need to navigate a large portion of the universe to decide is curvature. The distances and time needed is so large that we ourselves can’t observe it however, micro background photons can. In practice, curvature is measured by the size of the spots in microwave background maps (the larger the curvature the smaller the physical scale of the spots, we can use a globe to see this again).
“In a curved space, the light bends as it travels and acts like it is going through a lens. In a (positively) curved universe, a small object appears larger. If we know the actual size of an object, like a spot in the microwave background temperature due to sound waves, the size it appears lensed on the sky tells us the curvature of the universe”.
The curve decides the fate of the universe. What is curving space and lensing light is the matter itself. The more mass the universe has the greater its curvature due to the more mass. If the universe has enough mass, gravitational attraction can stop the expansion and cause the universe to recollapse.
“By measuring the curvature of the universe in the size of spots in the microwave background temperature maps, we determine the ultimate fate of the universe.
These are but two examples of the many things the microwave background can tell us about the origin, evolution and fate of the universe”.
It has been shown that microwave background radiation can be used to find the origins of structures in the universe. The M.B. can also help to understand the universe's fate. Einstein told us that matter curves space, the more the matter the more space is curved.
“Say my friend and I were in our tropical paradise near the equator. Paradise is boring he says, let's go north. We start out 100 feet apart and walk due north. Euclid told us that so long as we walk in perfectly straight lines, we should never meet, yet we all know that my friend and I will meet at the north pole. In my ignorance, I might say that there is a strange attractive force between my friend and I and label that "gravity". But in truth what happened was the surface of the earth curved beneath us as we walked”.
Using this example and similar examples, Einstein came to a revelation that gravity is no more than the curvature of spacetime. So curvature is created by matter itself.
It is unknown if the universe is a sphere, we need to navigate a large portion of the universe to decide is curvature. The distances and time needed is so large that we ourselves can’t observe it however, micro background photons can. In practice, curvature is measured by the size of the spots in microwave background maps (the larger the curvature the smaller the physical scale of the spots, we can use a globe to see this again).
“In a curved space, the light bends as it travels and acts like it is going through a lens. In a (positively) curved universe, a small object appears larger. If we know the actual size of an object, like a spot in the microwave background temperature due to sound waves, the size it appears lensed on the sky tells us the curvature of the universe”.
The curve decides the fate of the universe. What is curving space and lensing light is the matter itself. The more mass the universe has the greater its curvature due to the more mass. If the universe has enough mass, gravitational attraction can stop the expansion and cause the universe to recollapse.
“By measuring the curvature of the universe in the size of spots in the microwave background temperature maps, we determine the ultimate fate of the universe.
These are but two examples of the many things the microwave background can tell us about the origin, evolution and fate of the universe”.
Cosmic Inflation-
We are still trying to figure the origin of the large scale wrinkles. One theory is that a period of rapid expansion occurs is very small scale at the level of the particle soup and stretches them to cosmic size.
As time continues matter falls into these wrinkles and starts to accumulate to become heavier and heavier objects.
“The crucial period when this process of gravitational attraction and infall can occur is related to an important concept in cosmology called the horizon. Like the horizon on the earth, it is the point beyond which we're unable to look. Unlike the earth's horizon, this distance is increasing with time because light from more distant regions has had more time to reach us. Heuristically, if there is a large clump in the universe we only know to fall toward it once it comes into the horizon”.
As time continues matter falls into these wrinkles and starts to accumulate to become heavier and heavier objects.
“The crucial period when this process of gravitational attraction and infall can occur is related to an important concept in cosmology called the horizon. Like the horizon on the earth, it is the point beyond which we're unable to look. Unlike the earth's horizon, this distance is increasing with time because light from more distant regions has had more time to reach us. Heuristically, if there is a large clump in the universe we only know to fall toward it once it comes into the horizon”.
Temperature Sky Maps-
“A useful property of the microwave background is that when we look out across widely separated angles, we're looking at wrinkles on such large scales that this process of infall hasn't yet begun. We're looking at the primordial wrinkles themselves”.
The Music of Expansion-
Scientists believe that the fluctuations may have originated for a period of rapid expansion or inflation. Proof of this can be heard in the microwave background with the fundamental tone of a musical system related to its physical size (here the horizon size at least is scattering). Patterns are also developed with overtones at integer multiples of the fundamental frequency.
“In music, the pattern of overtones helps us distinguish one instrument from another: it is a kind of signature of the instrument that makes the sound. In the same way, the pattern of overtones in the sound spectrum of the microwave background ripples acts as a signature of inflation. Inflation's signatures are that the overtones follow a pure harmonic series with frequency ratios of 1:2:3”.
“In music, the pattern of overtones helps us distinguish one instrument from another: it is a kind of signature of the instrument that makes the sound. In the same way, the pattern of overtones in the sound spectrum of the microwave background ripples acts as a signature of inflation. Inflation's signatures are that the overtones follow a pure harmonic series with frequency ratios of 1:2:3”.