UT: Cosmology 101

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UT: Cosmology 101

Post by bystander » Fri Feb 18, 2011 8:43 pm

Cosmology 101: The Beginning
Universe Today | Vanessa D'Amico | 2011 Feb 17
How did the universe get its start? It’s one of the most pressing questions in cosmology, and likely one that will be around for a while. Here, I’ll begin by explaining what scientists think they know about the first formative seconds of the universe’s life. More than likely, the story isn’t quite what you might think.

In the beginning, there was… well, we don’t really know. One of the most prevalent misconceptions in cosmology is that the universe began as an immensely small, inconceivably dense collection of material that suddenly exploded, giving rise to space as we know it. There are a number of problems with this idea, not least of all the assumption implicit in an event termed the big “bang.” In truth, nothing “banged.” The notion of an explosion brings to mind an expanding tide of material, gradually filling the space around it; however, when our universe was born, there was no space. There was no time either. There was no vacuum. There was literally nothing.

Then the universe was born. Extremely high energies during the first 10-43 seconds of its life make it very difficult for scientists to determine anything conclusive about the origin of the cosmos. Of course, if cosmologists are correct about what they believe may have happened next, it doesn’t much matter. According to the theory of inflation, at about 10-36 seconds, the universe underwent a period of exponential expansion. In a matter of a few thousandths of a second, space inflated by a factor of about 1078, quickly separating what were once adjoining regions by unfathomable distances and blowing up tiny quantum fluctuations in the fabric of spacetime.

Inflation is an appealing theory for a number of reasons. First of all, it explains why we observe the universe to be homogeneous and isotropic on large scales – that is, it looks the same in all directions and to all observers. It also explains why the universe visually appears to be flat, rather than curved. Without inflation, a flat universe requires an extremely fine-tuned set of initial conditions; however, inflation turns this fine-tuning into a trick of scale. A familiar analogy: the ground under our feet appears to be flat (even though we know we live on a spherical planet) because we humans are so much smaller than the Earth. Likewise, the inflated universe is so enormous compared to our local field of view that it appears to be spatially flat.

As the theory goes, the end of inflation gave way to a universe that looked slightly more like the one we observe today. The vacuum energy that drove inflation suddenly transformed into a different kind of energy – the kind that could create elementary particles. At this point (only 10-32 seconds after the birth of the universe), the ambient temperature was still far too hot to build atoms or molecules from these particles; but as the seconds wore on, space expanded and cooled to the point where quarks could come together and form protons and neutrons. High-energy photons continued to dart around, continually striking and exciting charged protons and electrons.

So what happened next? How did this chaotic soup of matter and radiation become the vast expanse of organized structure that we see today? What’s going to happen to the universe in the future? And how do we know that this is the way the story unfolded? Make sure to check out the next few installments of Cosmology 101 for the answers to these questions and more!
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
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Ann
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Re: UT: Cosmology 101

Post by Ann » Wed Feb 23, 2011 9:41 am

That's nice. I hope this series will continue here.

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MalcolmP2

Re: UT: Cosmology 101

Post by MalcolmP2 » Wed Feb 23, 2011 11:22 pm

This has puzzled me for a long time :-

The "Inflation" descriptions/theory that I have read only address the 3 spatial Ds, but Einstein et al says that Time is intimatly linked into the 4D fabric. So did T inflate as well at the same er , time ,, ! er :) ?

Which brings me to another of my old puzzles :-

How do we know that 1 second now is the same interval as 1 second 'way back then'. In other words, perhaps time is not,, how to say ,, linear? perhaps even logarithmic and that the further back we look the longer things get and that there never was a beginning instant, the 'instant' just gets stretched way back to -infinity ?

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Re: UT: Cosmology 101

Post by Markus Schwarz » Mon Mar 07, 2011 3:00 pm

MalcolmP2 wrote:The "Inflation" descriptions/theory that I have read only address the 3 spatial Ds, but Einstein et al says that Time is intimatly linked into the 4D fabric.
The key is the astronomical observation, known as the Copernican principle, that the universe "looks and is the same everywhere". Once you combine that with Einstein's General Theory of Relativity, it allows you to split spacetime into space and time again. Phrased differently, it allows you to introduce a time coordinate all observers in the universe can agree on. This "universal time" does not inflate.
MalcolmP2 wrote:How do we know that 1 second now is the same interval as 1 second 'way back then?'
Since "universal time" does not inflate, one second of "universal time" now is and was one second "universal time" 'way back then'. But then, how do we know that our time on Earth is "universal time"? The answer lies in the cosmic microwave background radiation. This radiation fills the entire universe and has a very specific shape. Measuring it here on Earth, we see tiny variations of that shape. From these we can infer that we are speeding through the universe at a speed much less than the speed of light and, hence, that our time on Earth is very close to "universal time".
MalcolmP2 wrote: [T]he further back we look the longer things get and that there never was a beginning instant, the 'instant' just gets stretched way back to -infinity ?
As Hawking and others found out in the 60ies and 70ies, it does not take an infinite amount of time to reach the beginning 'instant'. Or the other way around: the 'instant' the universe was created in happened a finite while ago. The precise number depends on the matter contents of the universe, which has to be measured. The standard cosmological model gives 13.7 Gyr.

The big problem with the Big Bang, however, is that our currently known theories break down at that 'instant', and one hopes that a unification of quantum mechanics and general relativity can solve that puzzle. It may be that there is no real beginning. There also may have been one. But it is fair to say that Inflation (and all that came afterwards) happened a finite while ago.

I hope this answers your questions!

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UT: Cosmology 101

Post by bystander » Wed Mar 09, 2011 11:29 pm

Cosmology 101: The Present
Universe Today | Vanessa D'Amico | 2011 Mar 09
Welcome back! Last time, we discussed the first few controversial and eventful moments following the birth of our cosmos. Looking around us today, we know that in the span of just a few billion years, the universe was transformed from that blistering amalgam of tiny elementary particles into a vast and organized expanse just teeming with large-scale structure. How does something like that happen?

Let’s recap. When we left off, the universe was a chaotic soup of simple matter and radiation. A photon couldn’t travel very far without bumping into and being absorbed by a charged particle, exciting it and later being emitted, just to go through the cycle again. After about three minutes, the ambient temperature had cooled to such an extent that these charged particles (protons and electrons) could begin to come together and form stable nuclei.

But, despite the falling temperature, it was still hot enough for these nuclei to start to combine into heavier elements. For the next few minutes, the universe cooked up various isotopes of hydrogen, helium and lithium nuclei in a process commonly known as big bang nucleosynthesis. As time went on and the universe expanded even further, these nuclei slowly captured surrounding electrons until neutral atoms dominated the landscape. Finally, after about 300,000 years, photons could travel freely across the universe without charged particles getting in their way. The cosmic microwave background radiation that astronomers observe today is actually the relic light from that very moment, stretched over time due to the expansion of the universe.

If you look at a picture of the CMB (above), you will see a pattern of differently colored patches that represent anisotropies in the background temperature of the cosmos. These temperature differences originally stemmed from tiny quantum fluctuations that were dramatically blown up in the very early universe. Over the next few hundred million years, the slightly overdense regions in the spacetime fabric attracted more and more matter (both baryonic – the kind that you and I are made of – and dark) under the influence of gravity. Some small regions eventually became so hot and dense that they were able to begin nuclear fusion in their cores; thus, in a delicate dance between external gravity and internal pressure, the first stars were born. Gravity then continued its pull, dragging clumps of stars into galaxies and later, clumps of galaxies into galaxy clusters. Some massive stars collapsed into black holes. Others grew so heavy and bloated that they exploded, spewing chunks of metal-rich debris in every direction. About 4.7 billion years ago, some of this material found its way into orbit around one unassuming main sequence star, creating planets of all sizes, shapes, and compositions – our Solar System!

Billions of years of geology and evolution later, here we are. And there the rest of the universe is. It’s a pretty striking story. But what’s next? And how do we know that all of this theory is even close to correct? Make sure to come back next time to find out!
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

MalcolmP2

Re: UT: Cosmology 101

Post by MalcolmP2 » Sun Mar 13, 2011 12:25 am

Markus Schwarz wrote:
MalcolmP2 wrote:The "Inflation" descriptions/theory that I have read only address the 3 spatial Ds, but Einstein et al says that Time is intimatly linked into the 4D fabric.
... it allows you to split spacetime into space and time again. Phrased differently, it allows you to introduce a time coordinate all observers in the universe can agree on. This "universal time" does not inflate.
Thanks Markus for your help, I think I see where you are going.

But will have to think a bit about spliting time off again having only just got used to the idea of combining it in 4D with the other 3!
Obviously I am no cosmologist and since logarithmic time (or even inflationary time) is not a current theory amongst _real_! cosmologists I have to accept that it is not a "runner" but as I said in my previous post I am puzzled and, with your help, trying to get my head round the idea of an absolute starting point in a relative universe.

You say "a time coordinate all observers in the universe can agree on", does this mean all observers at all times of the universe, or all observers existing now ,,,, errr oooh dear I think I may have a problem defining 'now' ! My head hertz

Sorry, mods. please move if this is not the right place ,,
I hope this answers your questions!
Thank you, I am sure it does but at the moment I am too dumb to fully grasp it, I will think some more , :) and return later

best,
Malcolm.

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Re: UT: Cosmology 101

Post by bystander » Wed Mar 30, 2011 4:50 pm

Cosmology 101: The End
Universe Today | Vanessa D'Amico | 2011 Mar 29
[attachment=0]galaxies.jpg[/attachment]Full-sized image
Welcome back to the third, and last, installment of Cosmology 101. So far, we’ve covered the history of the universe up to the present moment. But what happens next? How will our universe end? And how can we be so sure that this is how the story unfolded?

Robert Frost once wrote, “Some say the world will end in fire; some say in ice.” Likewise, some scientists have postulated that the universe could die either a dramatic, cataclysmic death – either a “Big Rip” or a “Big Crunch” – or a slower, more gradual “Big Freeze”. The ultimate fate of our cosmos has a lot to do with its shape. If the universe were open, like a saddle, and the energy density of dark energy increased without bound, the expansion rate of the cosmos would eventually become so great that even atoms would be torn apart – a Big Rip. Conversely, if the universe were closed, like a sphere, and gravity’s strength trumped the influence of dark energy, the outward expansion of the cosmos would eventually come to a halt and reverse, collapsing on itself in a Big Crunch.

Despite the poetic beauty of fire, however, current observations favor an icy end to our universe – a Big Freeze. Scientists believe that we live in a spatially flat universe whose expansion is accelerating due to the presence of dark energy; however, the total energy density of the cosmos is most likely less than or equal to the so-called “critical density,” so there will be no Big Rip. Instead, the contents of the universe will eventually drift prohibitively far away from each other and heat and energy exchange will cease. The cosmos will have reached a state of maximum entropy, and no life will be able to survive. Depressing and a bit anti-climactic? Perhaps. But it probably won’t be perceptible until the universe is at least twice its current age.

At this point you might be screaming, “How do we know all this? Isn’t it all just rampant speculation?” Well, first of all, we know without a doubt that the universe is expanding. Astronomical observations consistently demonstrate that light from distant stars is always redshifted relative to us; that is, its wavelength has been stretched due to the expansion of the cosmos. This leads to two possibilities when you wind back the clock: either the expanding universe has always existed and is infinite in age, or it began expanding from a smaller version of itself at a specific time in the past and thus has a fixed age. For a long time, proponents of the Steady State Theory endorsed the former explanation. It wasn’t until Arno Penzias and Robert Wilson discovered the cosmic microwave background in 1965 that the big bang theory became the most accepted explanation for the origin of the universe.

Why? Something as large as our cosmos takes quite a while to cool completely. If the universe did, in fact, began with the kind of blistering energies that the big bang theory predicts, astronomers should still see some leftover heat today. And they do: a uniform 3K glow evenly dispersed at every point in the sky. Not only that – but WMAP and other satellites have observed tiny inhomogeneities in the CMB that precisely match the initial spectrum of quantum fluctuations predicted by the big bang theory.

What else? Take a look at the relative abundances of light elements in the universe. Remember that during the first few minutes of the cosmos’ young life, the ambient temperature was high enough for nuclear fusion to occur. The laws of thermodynamics and the relative density of baryons (i.e. protons and neutrons) together determine exactly how much deuterium (heavy hydrogen), helium and lithium could be formed at this time. As it turns out, there is far more helium (25%!) in our current universe than could be created by nucleosynthesis in the center of stars. Meanwhile, a hot early universe – like the one postulated by the big bang theory – gives rise to the exact proportions of light elements that scientists observe in the universe today.

But wait, there’s more. The distribution of large-scale structure in the universe can be mapped extremely well based solely on observed anisotropies in the CMB. Moreover, today’s large-scale structure looks very different from that at high redshift, implying a dynamic and evolving universe. Additionally, the age of the oldest stars appears to be consistent with the age of the cosmos given by the big bang theory. Like any theory, it has its weaknesses – for instance, the horizon problem or the flatness problem or the problems of dark energy and dark matter; but overall, astronomical observations match the predictions of the big bang theory far more closely than any rival idea. Until that changes, it seems as though the big bang theory is here to stay.
Attachments
A1689-zD1, one of the brightest and most distant galaxies, <br />is 12.8 billion light years away - an extremely far distance <br />in our expanding universe.<br />(Credit: NASA/ESA/JPL-Caltech/STScI)
A1689-zD1, one of the brightest and most distant galaxies,
is 12.8 billion light years away - an extremely far distance
in our expanding universe.
(Credit: NASA/ESA/JPL-Caltech/STScI)
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

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Re: UT: Cosmology 101

Post by owlice » Sun Apr 03, 2011 3:17 pm

Here's another cosmology tutorial for those who might be interested: http://www.astro.ucla.edu/~wright/cosmo_01.htm
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Re: UT: Cosmology 101

Post by dougettinger » Tue May 03, 2011 6:19 pm

MalcolmP2 wrote:
Markus Schwarz wrote:
MalcolmP2 wrote:The "Inflation" descriptions/theory that I have read only address the 3 spatial Ds, but Einstein et al says that Time is intimatly linked into the 4D fabric.
... it allows you to split spacetime into space and time again. Phrased differently, it allows you to introduce a time coordinate all observers in the universe can agree on. This "universal time" does not inflate.
Thanks Markus for your help, I think I see where you are going.

But will have to think a bit about spliting time off again having only just got used to the idea of combining it in 4D with the other 3!
Obviously I am no cosmologist and since logarithmic time (or even inflationary time) is not a current theory amongst _real_! cosmologists I have to accept that it is not a "runner" but as I said in my previous post I am puzzled and, with your help, trying to get my head round the idea of an absolute starting point in a relative universe.

You say "a time coordinate all observers in the universe can agree on", does this mean all observers at all times of the universe, or all observers existing now ,,,, errr oooh dear I think I may have a problem defining 'now' ! My head hertz
Sorry, mods. please move if this is not the right place ,
I hope this answers your questions!
Thank you, I am sure it does but at the moment I am too dumb to fully grasp it, I will think some more , :) and return later

best,
Malcolm.
Let's do a thought experiment. Let's say that some type of intelligent being could exist during each major phase of the Inflationary Period. These beings had to reside on existing objects. Let's assume that the communication between all things was still performed at the speed of light.

Assuming that quarks were produced almost at the beginning of Inflation, then time could be measured only with respect to the distances associated with the size of a quark. After protons were produced then time could be measured with respect to distances associated with the size of hydrogen and helium nuclei. When simple atoms were produced then time could be measured and only measured with respect to larger distances such as the orbital diameter or orbital circumference of an electron bonded to a proton. Time dimension would keep increasing as nucleosynthesis produced more and more complex atoms and isotopes. As the scale of things became larger and larger, the present intelligent being is now using years as a time unit, which is the orbit of earth around the sun.

In other words, time dimension becomes increasingly slower as the complexity of the universe increases. Perhaps time dimension represents the amount of entropy in the universe and the amount of 3D space that has been produced to date. These are my personal thoughts about time dimension. I pray I did not insult any cosmologists.
Doug Ettinger
Pittsburgh, PA

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