Couple of questions

[Bergen]

(ps. im norwegian so my spelling might be a bit off )

 

i started the university (computer science) last year. Outside the classes i have to take we have some optional classes we can take and beeing a hobby science nut i took a basic astronomy class.

 

First i have a problem with how the professor speaks of the big bang theory as undisputed fact. I get how we can speak of certain aspects of astronomy as solved but the origin of the universe ? really ? i havent heard anything about that ( that we solved the mystery ). I mean, i get how we can speak with certainty of how the aurora works, what the athmosphere on the different planets are like, how the sun works etc, becouse these things can be observed closely and even physicaly be interacted with. Obviously i get that we use many different tools to help us but imo we know waaay to little about the universe to speak of big bang as absolute truth. I think a good way to describe astronomy is by the tale about the blind men and the elephant:

 

 

The poem begins:

It was six men of Hindustan

To learning much inclined,

Who went to see the Elephant

(Though all of them were blind),

That each by observation

Might satisfy his mind

 

They conclude that the elephant is like a wall, snake, spear, tree, fan or rope, depending upon where they touch. They have a heated debate that does not come to physical violence. But in this version, the conflict is never resolved.

 

All in all it just seems like a hard profession to be in, in the way that if you have opposing views on a theory then your "no good" so to speak. When asking my professor about the bing bang not beeing an absolute certainty he dismisses it like mumbo jumbo and that they are "getting there" when it comes to the fact that much of the theory is not yet explained. To me that seems like a horrible way to do science, having the solution before you have the "equation" cant be good.

 

anyways, my second question is more of a spesific one.

 

 

There is alot of talk about there beeing matter in the universe and not just vacuum. That got me thinking of the shape of galaxies. They seem to swirl with all "arms" bent in the same direction as if they are encountering matter much as if you envision spinning a starfish in water. Is the shape of the galaxies due to some physics in the galaxy itself or does indeed matter interact with vacuum in this way ? and if there was indeed matter bending the galaxies wouldnt they halt and eventually stop as their momentum stopped ? Also IF the galaxy is swirling becouse of matter wouldnt that create immense pressure at the core of the galaxy and could that have anything to do with black holes ?

 

ps. im only taking a basic astronomy class so i obviously have "no idea" what im talking about lol , but if there are answers to this than please tell me about it.

 

thank you

Edited July 6, 2010 by Bergen

[Spyman]

A tale of two big bangs

 

Whenever you hear or read about cosmology, there is one distinction you should have in the back of your mind - otherwise, matters might get a bit confusing: The term "big bang" has two slightly different meanings, and the answer to questions like "Did the big bang really happen" depends crucially on which of the two big bangs you are talking about.

 

Did the big bang really happen? If you are talking about the big bang phase, the hot early universe as described by well-known physical theories (or, if you include inflation, by extrapolation from those theories), then there is good evidence that, yes, nearly 14 billion years ago, the cosmos developed in just the way described by the cosmological models (the main exhibits are the original abundances of light elements as deduced from astronomical observation, the distribution of far-away galaxies and the existence and properties of the so-called cosmic background radiation).

 

Whether or not there really was a big bang singularity is a totally different question. Most cosmologists would be very surprised if it turned out that our universe really did have an infinitely dense, infinitely hot, infinitely curved beginning. Commonly, the fact that a model predicts infinite values for some physical quantity indicates that the model is too simple and fails to include some crucial aspect of the real world.

 

Thus, while some cosmologists do not have a problem with assuming that our universe began in a singular state, most are convinced that the big bang singularity is an artefact - to be replaced by a more accurate description once quantum gravity research has made suitable progress. To be replaced with what? Nobody knows for sure. In some models, we can go infinitely far into the past (one example is presented in the spotlight text Avoiding the big bang). In others, the big bang is replaced by a beginning of the universe which avoids all infinities, but in which time itself is rather different from what we are used to (some more information about this can be found in the spotlight text Searching for the quantum beginning of the universe).

http://www.einstein-online.info/spotlights/big_bangs/?set_language=en

 

 

Galactic spiral arms

 

Originally, astronomers had the idea that the arms of a spiral galaxy were material. However, if this were the case, then the arms would become more and more tightly wound, since the matter nearer to the center of the galaxy rotates faster than the matter at the edge of the galaxy. The arms would become indistinguishable from the rest of the galaxy after only a few orbits. This is called the winding problem.

 

Lin and Shu proposed in 1964 that the arms were not material in nature, but instead made up of areas of greater density, similar to a traffic jam on a highway. The cars move through the traffic jam: the density of cars increases in the middle of it. The traffic jam itself, however, does not move (or not a great deal, in comparison to the cars). In the galaxy, stars, gas, dust, and other components move through the density waves, are compressed, and then move out of them.

 

More specifically, the density wave theory argues that the "gravitational attraction between stars at different radii" prevents the so-called winding problem, and actually maintains the spiral pattern

http://en.wikipedia.org/wiki/Density_wave_theory

[Bergen]

A tale of two big bangs

 

Whenever you hear or read about cosmology, there is one distinction you should have in the back of your mind - otherwise, matters might get a bit confusing: The term "big bang" has two slightly different meanings, and the answer to questions like "Did the big bang really happen" depends crucially on which of the two big bangs you are talking about.

 

Did the big bang really happen? If you are talking about the big bang phase, the hot early universe as described by well-known physical theories (or, if you include inflation, by extrapolation from those theories), then there is good evidence that, yes, nearly 14 billion years ago, the cosmos developed in just the way described by the cosmological models (the main exhibits are the original abundances of light elements as deduced from astronomical observation, the distribution of far-away galaxies and the existence and properties of the so-called cosmic background radiation).

 

Whether or not there really was a big bang singularity is a totally different question. Most cosmologists would be very surprised if it turned out that our universe really did have an infinitely dense, infinitely hot, infinitely curved beginning. Commonly, the fact that a model predicts infinite values for some physical quantity indicates that the model is too simple and fails to include some crucial aspect of the real world.

 

Thus, while some cosmologists do not have a problem with assuming that our universe began in a singular state, most are convinced that the big bang singularity is an artefact - to be replaced by a more accurate description once quantum gravity research has made suitable progress. To be replaced with what? Nobody knows for sure. In some models, we can go infinitely far into the past (one example is presented in the spotlight text Avoiding the big bang). In others, the big bang is replaced by a beginning of the universe which avoids all infinities, but in which time itself is rather different from what we are used to (some more information about this can be found in the spotlight text Searching for the quantum beginning of the universe).

http://www.einstein-online.info/spotlights/big_bangs/?set_language=en

 

my problem wasnt with the THEORY of big bang itself but with the certainty that my professor spoke of it as the alfa of the universe.

 

Galactic spiral arms

 

Originally, astronomers had the idea that the arms of a spiral galaxy were material. However, if this were the case, then the arms would become more and more tightly wound, since the matter nearer to the center of the galaxy rotates faster than the matter at the edge of the galaxy. The arms would become indistinguishable from the rest of the galaxy after only a few orbits. This is called the winding problem.

 

Lin and Shu proposed in 1964 that the arms were not material in nature, but instead made up of areas of greater density, similar to a traffic jam on a highway. The cars move through the traffic jam: the density of cars increases in the middle of it. The traffic jam itself, however, does not move (or not a great deal, in comparison to the cars). In the galaxy, stars, gas, dust, and other components move through the density waves, are compressed, and then move out of them.

 

More specifically, the density wave theory argues that the "gravitational attraction between stars at different radii" prevents the so-called winding problem, and actually maintains the spiral pattern

http://en.wikipedia.org/wiki/Density_wave_theory

 

reading the wiki page you provided made it clear to me that i cant really discuss further on this as the answers seem to be at a high level in astronomy that i have yet to learn. One thing tho, does this theory assume there is only vacuum ? becouse if there is indeed some matter in the "vastness" then this would surely impact the shape and physics of the galaxy itself, no ?

[crashonly]

To adress your first question (a bit):

 

Here's a link to a brilliant talk by a brilliant physicist Lawrence Krauss, entitled

. He talks quite a bit about how exactly it was that we found out that the universe was expanding, and how do we trace the expansion further and further back in time. He talks in lay terms, so that even a computer scientist (I know, as I am a student of CS myself ) will understand!

 

EDIT: Oh, and feel free to skip through the introduction part by Richard Dawkins.

[Spyman]

my problem wasnt with the THEORY of big bang itself but with the certainty that my professor spoke of it as the alfa of the universe.

Well, your professor is correct then, since the ONLY theory we have that is able to explain all our observations of the Universe depends on a metric expansion of space. There are currently no serious alternatives considered among the scientific community, so it is the "alfa" model of the Universe.

 

The Big Bang

The Big Bang is the prevailing cosmological theory of the early development of the universe. Cosmologists use the term Big Bang to refer to the idea that the universe was originally extremely hot and dense at some finite time in the past and has since cooled by expanding to the present diluted state and continues to expand today. The theory is supported by the most comprehensive and accurate explanations from current scientific evidence and observation.

http://en.wikipedia.org/wiki/Big_Bang

 

Observational evidence

Theoretical cosmologists developing models of the universe have drawn upon a small number of reasonable assumptions in their work. These workings have led to models in which the metric expansion of space is a likely feature of the universe. Chief among the underlying principles that result in models including metric expansion as a feature are:

 

- the Cosmological Principle which demands that the universe looks the same way in all directions (isotropic) and has roughly the same smooth mixture of material (homogeneous).

- the Copernican Principle which demands that no place in the universe is preferred (that is, the universe has no "starting point").

 

Scientists have tested carefully whether these assumptions are valid and borne out by observation. Observational cosmologists have discovered evidence - very strong in some cases - that supports these assumptions, and as a result, metric expansion of space is considered by cosmologists to be an observed feature on the basis that although we cannot see it directly, scientists have tested the properties of the universe and observation provides compelling confirmation. Sources of this confidence and confirmation include:

 

- Hubble demonstrated that all galaxies and distant astronomical objects were moving away from us, as predicted by a universal expansion. Using the redshift of their electromagnetic spectra to determine the distance and speed of remote objects in space, he showed that all objects are moving away from us, and that their speed is proportional to their distance, a feature of metric expansion. Further studies have since shown the expansion to be extremely isotropic and homogeneous, that is, it does not seem to have a special point as a "center", but appears universal and independent of any fixed central point.

- In studies of large-scale structure of the cosmos taken from redshift surveys a so-called "End of Greatness" was discovered at the largest scales of the universe. Until these scales were surveyed, the universe appeared "lumpy" with clumps of galaxy clusters and superclusters and filaments which were anything but isotropic and homogeneous. This lumpiness disappears into a smooth distribution of galaxies at the largest scales.

- The isotropic distribution across the sky of distant gamma-ray bursts and supernovae is another confirmation of the Cosmological Principle.

- The Copernican Principle was not truly tested on a cosmological scale until measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems were made. A group of astronomers at the European Southern Observatory noticed, by measuring the temperature of a distant intergalactic cloud in thermal equilibrium with the cosmic microwave background, that the radiation from the Big Bang was demonstrably warmer at earlier times. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the universe is experiencing a metric expansion.

 

Taken together, the only theory which coherently explains these phenomena relies on space expanding through a change in metric. Interestingly, it was not until the discovery in the year 2000 of direct observational evidence for the changing temperature of the cosmic microwave background that more bizarre constructions could be ruled out. Until that time, it was based purely on an assumption that the universe did not behave as one with the Milky Way sitting at the middle of a fixed-metric with a universal explosion of galaxies in all directions (as seen in, for example, an early model proposed by Milne). Yet before this evidence, many rejected the Milne viewpoint based on the Mediocrity principle.

 

Additionally, scientists are confident that the theories which rely on the metric expansion of space are correct because they have passed the rigorous standards of the scientific method. In particular, when physics calculations are performed based upon the current theories (including metric expansion), they appear to give results and predictions which, in general, agree extremely closely with both astrophysical and particle physics observations. The spatial and temporal universality of physical laws was until very recently taken as a fundamental philosophical assumption that is now tested to the observational limits of time and space. This evidence is taken very seriously because the level of detail and the sheer quantity of measurements which the theories predict can be shown to precisely and accurately match visible reality. The level of precision is difficult to quantify, but is on the order of the precision seen in the physical constants that govern the physics of the universe.

http://en.wikipedia.org/wiki/Metric_expansion_of_space

 

 

One thing tho, does this theory assume there is only vacuum ? becouse if there is indeed some matter in the "vastness" then this would surely impact the shape and physics of the galaxy itself, no ?

While space is not totally empty it is still very close to a vacuum, which means that there are very very little matter in the voids outside galaxies. It follows that very little matter also has a very small impact and can thus be neglected. Even something as big and massive as a vandering star that happens to get to close, would simply be dragged along and forced to join the galaxy and its rotation, without the galaxy slowing down any measurable level.

 

Outer space (often simply called space) is the void that exists beyond any celestial body including the Earth. It is not completely empty (i.e. a perfect vacuum), but contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation, magnetic fields, and neutrinos.

 

Intergalactic space is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a total vacuum. The space between galaxy clusters, called the voids, is probably nearly empty. Some theories put the average density of the Universe as the equivalent of one hydrogen atom per cubic meter. The density of the universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as planets, stars, and black holes) to conditions in vast voids that have much lower density than the universe's average.

 

Surrounding and stretching between galaxies, there is a rarefied plasma that is thought to possess a cosmic filamentary structure and that is slightly denser than the average density in the universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the universe in rich clusters of galaxies.

http://en.wikipedia.org/wiki/Outer_space

Edited July 7, 2010 by Spyman
Adding Big Bang quote

[ewmon]

Bergen, do you mean to ask how accurately/confidently can scientists use a couple hundred years of observations to estimate what happened over the past 14 billion years — that is, reconstruct the whole thing based on one millionth of one percent of it?

 

Myself, I would say that any such theory is almost guaranteed to contain serious flaws, and because it can never be repeated, we'll never know in a true scientific sense.