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03 Feb 2019 00:56

Watsisname wrote:
Source of the post Dark matter is mysterious, but it is not that complicated.

If it was simple within the framework of cosmology, then why hasn't it's exact nature been identified yet? If our idea of the cosmos is correct within reasonable certainty (i.e. predictable), then dark matter should be accountable. By stating it is mysterious, this either implies that something is amiss with our theories, or that this phenomena has operations beyond our current understanding of the universe.
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03 Feb 2019 01:20

Stellarator wrote:
Source of the post If it was simple within the framework of cosmology, then why hasn't it's exact nature been identified yet?

We don't know what it is "made of" (assuming it isn't fundamental, like electrons according to our current understanding), and we don't know the individual masses of the particles (if it is particles and not, say, a fluid).  Answering those questions would require a much more direct form of measurement on the substance, which is difficult to do because it is weakly interacting.

However, its dynamical properties -- how it affects the motions of stars and galaxies and the evolution of the universe -- are very simple.  The way we treat it is as a collection of non-relativistic (speeds much less than light) particles with significant rest masses, but little or no interaction with itself or other matter except by gravity.  This model has done an outstanding job of both explaining old observations and predicting new ones.

One way to think about dark matter's properties is to imagine combining certain properties of particles that we already know about.  Neutrons are massive and lack electric charge, and neutrinos are weakly interacting.  A population of particles in the universe that combines those properties would behave a lot like dark matter -- being nearly impossible to detect except for its gravitating effects on other matter.
 
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03 Feb 2019 01:58

Watsisname wrote:
Source of the post One way to think about dark matter's properties is to imagine combining certain properties of particles that we already know about.  Neutrons are massive and lack electric charge, and neutrinos are weakly interacting.  A population of particles in the universe that combines those properties would behave a lot like dark matter -- being nearly impossible to detect except for its gravitating effects on other matter.

I was just about to ask you if was likely that dark matter was a combination of components! Care to expand on this? What would be the exact properties of these particles and how might we eventually detect them? What might be some alternatives to particles who's net effect resulted in this interaction we call dark matter?
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03 Feb 2019 08:17

Hi again, and I have a QUESTION.
It's about the planet called CoRoT-Exo-7b and about the chthonian planets in general: how can an Earth-sised and Earth-massed core hold so much gas? Any ideas?
 
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03 Feb 2019 12:12

How much gas it can hold depends not only on size and mass, but also what kind of gas and temperature.
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07 Feb 2019 00:30

Just today, I was reading about the 'Great Attractor' gravitational anomaly found to be embedded within the newly named Laniakea Galaxy Supercluster(a cluster that includes our own Local Galaxy Group along with many others). An odd question popped into my head: Since the Laniakea galaxies are being pulled from their relative trajectories towards this Great Attractor - could a similar gravitational anomaly of far greater mass hypothetically pull all the galaxies of the known universe to its core? Before I go on, I understand that the Great Attractor is most likely a supermassive clump of galaxies, so 'core' is a bit of a misnomer - but would such a hypothetical mass defy the expansion of the universe to such a degree that, despite the space still expanding between the stars and galaxies, the galaxies within Attractor wouldn't lose photonic sight of one another in the far, far future as the universe expands as per Hubble's Law?
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07 Feb 2019 10:14

Another expansion thought here. The universe is expanding, we see light from distant objects redshifted from this expansion. I'm assuming the universe can expand forever, under certain conditions. So, my question, can the wavelengths of Lightwaves also expand or stretch infinitely? Way beyond radio waves, what happens when the wave is longer than the speed of light?
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07 Feb 2019 17:20

Stellarator wrote:
Source of the post What would be the exact properties of these particles and how might we eventually detect them? What might be some alternatives to particles who's net effect resulted in this interaction we call dark matter?

In brief, dark matter must have the following properties:

  • Its component particles must be massive.
  • They must be non-relativistic (speeds much slower than light).
  • They must have little or no electric charge.
  • They must be weakly interacting except by gravity.
    
First let's go over the reasons they must have these properties.  The most obvious is that they must be massive, because they must explain the observations indicating significant additional gravitational mass in the universe.

Why must they move slowly?  If they moved too quickly, then they wouldn't be able to clump together, and would instead uniformly fill space.  But by observations of gravitational lensing, we know the dark matter is clumpy, on the scales of galaxies and clusters of galaxies.  We call these structures "dark matter halos" that the galaxies and clusters are embedded in.

The lack of electric charge is required in order for the dark matter to be, well, invisible.  If it had charge, then it would interact with electromagnetic fields and photons, and we would see it by absorption, emission, or scattering of light.  Charge (even a little of it) would also cause the dark matter to be deflected by the electric or magnetic fields around galaxies, which we know is not the case.

Finally there's the weak interaction.  Dark matter particles must be unable to interact with each other or with other particles except by gravitation, and possibly also the weak nuclear force.  

What particles do we know of that possess some of these characteristics?  As I mentioned, neutrons are both massive and electrically neutral.  But they have short lifespans (about 10 minutes on average) when not confined in atomic nuclei.  Free neutrons are unstable.  They also interact by the strong force, so they readily interact with atomic nuclei, and are absorbed easily by lightweight nuclei.  This is actually a pretty neat property -- a beam of neutrons may pass easily through lead, but be absorbed strongly by hydrogen atoms in water.  This is almost the opposite of X-ray imaging: neutron beam can be used to make a finely detailed image of a flower, completely enclosed in a lead container:

Image

As I also mentioned, neutrinos have the property of being weakly interacting.  They can pass easily through an entire planet.  A light year of lead would be needed to absorb half of them.  It might be tempting then to think that neutrinos could explain dark matter.  The problem with them is they are too fast (highly relativistic) and have very small rest masses.  Possibly, some of the dark matter is a population of some exotic type of "sterile" neutrino which is slower and more massive.

What other type of particle might be consistent with dark matter?  Actually there are many hypotheses, but let's look at one that's fairly simple and, at a glance, intuitive.  How about a population of primordial black holes, formed just after the Big Bang?

Black holes evaporate (and more rapidly, the smaller they are), so they must be at least 2x1011kg to survive since the Big Bang.  What if we imagine the universe contains a bunch of black holes weighing around 1015 kg?  A black hole with this mass would live about 2x1021 years, emitting (today) about 350 Watts of power, mostly as hard X-ray or soft gamma ray photons with about 50keV energy.  The event horizon radius is smaller than a hydrogen atom (about 1.5x1012 m), and the distance at which its gravitational pull is less than 1g is about 80 meters.  

Sounds good so far -- the effects of these black holes would be nearly unobservable over astronomical distances, except by how they gravitate collectively.  But it probably would be noticeable if one happened to pass through Earth.  How likely is that?  Let's compute how many black holes of this mass would we need and how many per volume of space that is.  Multiply the critical density of the universe by the fraction which is dark matter, and divide that by 1015 kg per dark matter particle as a black hole, and we get 1 such black hole on average within a sphere of radius 300AU.  Would one of these black holes ever intersect Earth?  Probably no!  Moving on average a few hundred to thousand km/s relative to us, it would take at least 100 times the age of the universe for one to intersect us!

So at a glance, primordial black holes (if these even exist) seems like a promising candidate for explaining dark matter.  The problem with the idea is that there is only a very narrow range of possible masses that can work, which makes it a possibility that is already highly constrained against.  They can't be too small, or they'd have already evaporated, or we'd see a lot of evaporating ones now.  If they're too small then there must also be more of them and we expect more intersections between them and Earth or other objects.  On the other hand if they are too big, then although there will be fewer of them, we would expect to see their gravitational effects (at an individual level), especially in more densely populated areas like galactic centers.
 
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07 Feb 2019 18:00

Thank you for the elaboration, it has greatly helped my understanding of dark matter.

Watsisname wrote:
Source of the post So at a glance, primordial black holes (if these even exist) seems like a promising candidate for explaining dark matter.

An interesting theory, do any cosmological observations agree with it?
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08 Feb 2019 02:46

Stellarator wrote:
Source of the post would such a hypothetical mass defy the expansion of the universe to such a degree that, despite the space still expanding between the stars and galaxies, the galaxies within Attractor wouldn't lose photonic sight of one another in the far, far future as the universe expands as per Hubble's Law?

Yes, it does.  On very large scales the distribution of matter in the universe appears uniform, and the space undergoes metric expansion.  But on smaller scales, where the distribution of matter appears less uniform, regions that are denser collapse under their own gravity.  Stars within galaxies, and even galaxies within clusters, have motions described by that local gravitational field, rather than the cosmological expansion, so these galaxies never get pulled apart.

If dark energy remains constant (as in the cosmological constant), which we expect it to, then in the very distant future the proper distance to the cosmic event horizon will be about 17 billion light years.  That then will be the maximum possible size of any gravitationally bound system in the universe.  Right now, the maximum size of bound clusters is a few hundred million light years, however measuring the exact value is tricky.

Stellarator wrote:
Source of the post Attractor - could a similar gravitational anomaly of far greater mass hypothetically pull all the galaxies of the known universe to its core?


No, for a subtle reason.  The observable universe is much larger (46 billion light years in proper distance) than the distance to the cosmological event horizon (about 16 billion light years).  We receive light (emitted a long time ago) from galaxies that are now beyond that event horizon, but we cannot send signals to them now and have those signals ever reach them.  Likewise, we cannot reach them ourselves, nor can their gravitation ever pull us there.  The space in between is expanding too quickly.  

If we imagine some very massive cluster (or something) was beyond our cosmic event horizon, then we would have some left-over velocity towards it due to it being inside of our cosmic event horizon in earlier times, during which it could influence us.  This is a potential explanation for "dark flow".  But again we'll never meet that source, nor will any other galaxies that have slipped beyond its cosmic horizon.

If we imagine further that there was some stupendously massive object, whose mass made up the majority of the observable universe, then this would change the geometry of the space time.  Its gravitational field would dominate over the expansion everywhere, and everything would fall into it.

Stellarator wrote:
Source of the post An interesting theory, do any cosmological observations agree with it? 

It has been tested in a few ways, but observations do not so much "agree" as rule out certain mass distributions such a populations of black holes could have.  This has actually been so tightly constrained now that some authors are saying we can be confident that at least half of the dark matter cannot be explained by primordial black holes, because of upper limits on their numbers now placed by observations of gravitational microlensing events.  Basically, if there are many of these black holes, then we expect them to occasionally pass in front of distant supernovae and produce an anomalous brightening.  We don't see any signs of that.

Dark matter as primordial black holes is a popular hypothesis going back decades, but it's growing weaker as more and more "possibility space" for such black holes is ruled out by observations.  It also has so far not had any predictive successes.  But we've only grown more confident that dark matter, as some sort of massive weakly interacting particles, does exist.

Gnargenox wrote:
Source of the post So, my question, can the wavelengths of Lightwaves also expand or stretch infinitely?

In principle there is no upper limit to the wavelength of a photon.  This is because their sources can oscillate with arbitrarily low frequency, and the Planck distribution has no lower bound.  And there is also no limit to how much a photon can be stretched.  Black hole horizons, the cosmological horizon, and even the expansion of the universe itself, may generate infinite redshift.

Infinite redshift of a photon of course means their energy trends to zero.  But it also means the photons are "delocalized", in the sense that if an instrument could detect them (as photon strikes), then they could be detected anywhere, because the wave function for that photon will spread over all of space.  The catch is the size of the detector must also trend to infinity to have any fair chance of finding it.

All of that is theory, but in practice there's no way for arbitrarily long photons to exist.  Let's use emission from black holes as an example.

Main idea: The closer to the event horizon a photon is emitted from, the longer its wavelength will be when detected far away.  Seems straightforward.  However, we cannot say with arbitrary precision how close to the event horizon the photon was emitted from.  The uncertainty principle tells us that in order to constrain the distance from the horizon to Δx, the uncertainty in the momentum (in this case the momentum of the photon) must be at least ℏ/(2Δx).  The momentum of a photon is p = E/c, and a photon's energy and wavelength are related by E = hc/λ.  Go through the algebra, and we find that the wavelength (in the source frame) must be smaller than 4πΔx.  What a simple and logical result!  The closer to the horizon the photon was emitted from, the shorter its wavelength must be, and they follow a direct proportionality.  The wavelength of the light is itself the limit for the measurement.

(Aside: we can reach the exact same result by going through the energy-time uncertainty principle, and arguing that the time uncertainty, Δt, must be Δx/c.  Also we may as well throw away the factor of 4π and say that the wavelength is equal to the distance Δx.)

Now we'll compute how much this photon of wavelength λ will be redshifted as it climbs away to infinity, from its source at Δx from the horizon.  For a fixed distance from the horizon, this becomes more extreme for larger mass black holes.  If we consider the black hole SgrA* with 4 million solar masses, then photons we know to have been emitted from within 1 meter above the horizon must have a wavelength (in source frame) shorter than about 1 meter.  The gravitational redshift factor 1m above the horizon is around 100,000, so the photon observed very far away must be shorter than about 100km.  Counter-intuitively, the smaller we make Δx here, the smaller the wavelength will be when seen far away.  At 1cm from the horizon, the maximum wavelength when detected far away will be about 10km.

This sounds very backwards from what we're usually taught about how light emitted closer to the horizon is redshifted more.  But that assumed the light could have an arbitrary wavelength at the source.  The uncertainty principle forbids us from saying a 1m wavelength photon was emitted 1cm above the horizon.  That wouldn't even make sense logically -- is 99% of that photon below the horizon?  Or 99% at a distance greater than 1cm?  Both lead to contradictions with saying the photon was observed at all, or with such a great redshift.

This (admittedly very simple and handwavy) argument using black hole horizons also generalizes fairly well for any sort of infinite redshift we may imagine.  For cosmic expansion, the limit is placed by the size of the cosmological horizon and the amount of time the universe has existed.
 
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11 Feb 2019 11:01

I have been invite to explain my reluctance to believe the theory of the "big bang."

My main problem with it has to do with mathematical limits. In our science of mathematics you sooner or later will come across the concept of mathematical limits. A limit is basically a point where our mathematics breaks down, and in order to maintain it, we need to set limits. This may be an over simplification, but I believe the reason that limits are necessary is because our mathematics is still in its infancy - we still have alot to discover, hence our science has holes in it, to be blunt.

I am no expert by any means on this topic and if my memory serves me correctly, a mathematical limit is a point which can never be reached, it can only be approached, since it tends to infinity. The big bang theory has all the hallmarks of a limit to me. We always hear about our astronomy being able to see a point in time ever closer to TIME=0, but never the moment itself. I don't expect we will ever see this point as I don't believe it to be true.

My objection to the bang, is the point just one second before it? How can we get something out of nothing? A whole universe? Sounds shaky to me. What was the universe one second before the big bang? There is also evidence that the expansion of the universe is decelerating and will possibly go the other way - to start contracting? I don't have a problem imagining the universe expanding, but if you take that thought to its ultimate conclusion, it is the problem of the moment it is supposed to have all begun, when the universe was infinitessimally small, which I have trouble with. It borders on religion. Our maths has a simple solution - we create a limit.

On the subject of scientific theories and their possible errors I have no problem being skeptical of many theories. The law of gravity is becoming increasingly shaky as this millenia progresses. It does not mean that our law is 100 per cent wrong. It just means that it is not 100 percent correct and we need to discover the missing parts to it. F=ma is not as universally correct as we once thought it was. I would say it should be F=ma + X. X has still not fully revealed itself.

I welcome any one's counter thoughts to my own.
 
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11 Feb 2019 12:13

Hi HKATER, I hope I can answer some of these questions and clarify any misunderstandings for you.  I am an astrophysics major and I do research related to cosmology.

HKATER wrote:
Source of the post I am no expert by any means on this topic and if my memory serves me correctly, a mathematical limit is a point which can never be reached, it can only be approached, since it tends to infinity. The big bang theory has all the hallmarks of a limit to me. We always hear about our astronomy being able to see a point in time ever closer to TIME=0, but never the moment itself. I don't expect we will ever see this point as I don't believe it to be true.


Yes, our understanding of the early universe acts a lot like a limit.  The moment "t=0" is rather an extrapolation of the expansion backwards to when the size or scale factor of the universe would be zero.  

Such extrapolation of course cannot work at t=0 because quantities like temperature and energy density would be infinite.  But what we're interested in, and what we can test, are what the conditions were like very close to it.  The Big Bang theory is about what the conditions were like at very short times after t=0, and how the universe has evolved from those conditions to become what we observe today.  In simple terms, the universe was very hot and dense at early times, and it has expanded and cooled ever since.  We can apply math and physics to the expansion to make predictions about what we should observe as we look farther outward in space (and thus farther back in time), so this becomes a testable model for understanding the universe.  

An analogy can be made to the electric field of a particle.  Classically, electrons are point particles, and the electric field -- following the inverse square law -- would be infinite at that point.  Of course we know such a description must break down at those points themselves because physics doesn't work well when quantities become infinite, like singularities.  But this "inverse square law field from a point" model works extremely well for describing how the electric field behaves around and very close to those points.  


HKATER wrote:
Source of the post My objection to the bang, is the point just one second before it? How can we get something out of nothing?

Just as we don't know what happens exactly at t=0, we don't know what preceded it.  When some scientists describe "everything coming from nothing", they are referring to how matter is created from energy.  At very early times the temperature and energy density were great enough to produce all kinds of particles "out of nothing", but what "nothing" really means here is vacuum, and the vacuum is a dynamic thing where lots of things can happen.

As crazy as that sounds, this is something we can do in the lab. Photons are not matter, but oscillations of electric and magnetic field, which are vacuum.  But collide sufficiently high energy photons together, and you can create particles of matter!

HKATER wrote:
Source of the post There is also evidence that the expansion of the universe is decelerating and will possibly go the other way - to start contracting?

I don't think that's right -- all observations I'm aware of are consistent with accelerating expansion. :)  But even if it was decelerating this wouldn't contradict the Big Bang.  For example, a universe filled with a very high amount of matter would start out expanding, but the expansion rate would slow and reverse and then eventually collapse in a Big Crunch.  I show this in detail here for a collapsing universe:

http://forum.spaceengine.org/viewtopic.php?t=47&start=1110#p23833

Observations are very clear that our universe will not collapse, but instead undergo exponential expansion.  This is understood as the "Lambda-CDM" model, which I also describe in that post.  


HKATER wrote:
Source of the post The law of gravity is becoming increasingly shaky as this millenia progresses. It does not mean that our law is 100 per cent wrong. It just means that it is not 100 percent correct and we need to discover the missing parts to it. F=ma is not as universally correct as we once thought it was. I would say it should be F=ma + X. X has still not fully revealed itself.

F=ma is a Newtonian formula.  It works extremely well under conditions where gravity is not too strong, distances are not too small, and speeds are not too fast.  In other words, it is perfectly adequate for most conditions you are likely to encounter in every day life.  

But if speeds are close to the speed of light, then we use special relativity.  If gravity is very strong, then we use general relativity.  And on very small scales, we use quantum mechanics.  But it is important to understand that relativity and quantum mechanics all simplify to Newtonian mechanics in those respective limits.  Let speeds and masses be small in general relativity, and you get back Newton's Laws.

Also F=ma is somewhat a limited case even in Newtonian physics.  The more general formula is F = dp/dt, meaning that force is the rate at which momentum is changing.  Momentum is mass times velocity (mv), but if m is fixed then you get m(dv/dt) which is ma. :)
 
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11 Feb 2019 13:06

HKATER wrote:
Source of the post  A limit is basically a point where our mathematics breaks down, and in order to maintain it, we need to set limits.

Zeno argued similarly nearly 2500 years ago.  He argued that a faster runner can never overtake a slower runner, because in order to do so, the faster runner must first reach the position of the slower runner, and when he's there, he must reach the position that the slower runner has reached meanwhile, and so on.  Clearly, there is a limit here.  But this doesn't mean that "our mathematics breaks down".
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11 Feb 2019 23:17

midtskogen wrote:
Source of the post Clearly, there is a limit here.  But this doesn't mean that "our mathematics breaks down".

Great example.  In Zeno's case, this is the kind of limit that is solved by the development of calculus.  Calculus is perhaps the greatest achievement in all of mathematics, as it enables us to analyze situations in which things are changing and when the rates themselves are changing, or even if some rates of change depend on how other rates of change are changing.  (What a mouthful).  Such situations are ubiquitous in nature.

The type of limit posed by the first moment of expansion in the Big Bang is a problem not so much of mathematics, but of our understanding of physics.  The expansion is described by general relativity, and general relativity predicts the expansion traces all the way back to singularity state of infinite density at "t=0".  Infinite density is a problem.  We have no physics that can deal with infinite densities.  More likely what this prediction is really telling us is that general relativity is not an accurate theory when conditions get too extreme.  We'll need a theory of quantum gravitation at the very least, and what can help get us there are experiments in future generations of particle accelerators.

That being said, despite this breakdown of general relativity close to the Big Bang, it is still accurate within a fraction of a second of what we would consider t=0.  We understand what the universe was like and how it has changed since it was 0.0000000000000001% of its current age.  To put that in perspective, that would be like measuring the distance from the Earth to the Moon and being accurate to 1 nanometer.
 
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12 Feb 2019 01:34

Watsisname wrote:
Source of the post
   HKATER wrote:
   My objection to the bang, is the point just one second before it? How can we get something out of nothing?


Just as we don't know what happens exactly at t=0, we don't know what preceded it.  When some scientists describe "everything coming from nothing", they are referring to how matter is created from energy.  At very early times the temperature and energy density were great enough to produce all kinds of particles "out of nothing", but what "nothing" really means here is vacuum, and the vacuum is a dynamic thing where lots of things can happen.

As crazy as that sounds, this is something we can do in the lab. Photons are not matter, but oscillations of electric and magnetic field, which are vacuum.  But collide sufficiently high energy photons together, and you can create particles of matter!

A good point Wats, may I elaborate? HKATER, you are correct in specifically asking "How" we get something out of nothing - because all to often people will instead ask "why" - which isn't very scientific and usually implies that there is some underlying reason or method to the universe. There is not - but you will get a much more concise answer if you know "how" something is. 

The fact is that in physics there is no such thing as "nothing". No field of a particle (particles are excitation's of fields) can equal zero energy. Even in a perfect vacuum, virtual particles and fields can pop in and out of existence on a quantum level. These so-called vacuum fluctuations can, over infinite time, create matter via electromagnetic interaction from the apparent 'nothingness' (this is a very basic simplification of things). In case you are interested, the spontaneous appearances of virtual particles can be demonstrated in a lab. So to say that "something was made out of nothing" isn't really correct.

In our modern universe, interactions between virtual particles can only occur on the subatomic level. However, during the expansion of the universe, these were stretched and exaggerated due to hyper-acceleration by dark energy into densities of mass, thus giving birth to galaxies, stars and eventually planets. The catalyst for the hyper-acceleration of the universe is called Inflation theory. In the end, the imprint of these vacuum fluctuations can be seen in the Cosmic Microwave Background radiation (CMB).

To elaborate, current theories in cosmology don't rely on the concept of 'nothing' to predate the universe. There are many pre-origin theories made by top scientists in the field that can be plausible within a mathematical framework:

There is Eternal Inflation, Stephen Hawking's black-hole multiverse model, Heat Death Quantum fluctuation universe cycle and the Big Bounce Universe Cycle, just to name a few. Each could be true and sound. All could be wrong. One or none of the above theories could be vindicated when we can solidify our understanding of the density of the universe, it's topological curvature of space/time and the true value of Hubble's Law (among other physics mysteries) and ultimately conclude with a Grand Unified Field Theory and Theory of Everything. We simply need more data and a further understanding of the universe. Learning more about them never did anyone any harm though.


If the ideas presented here all seem rather arcane to you, I'd encourage you to ask for clarification here on the forum. Watsisname does an excellent job of explaining such things, but as additional reading, I would also point you to the book "A Universe from Nothing" by physicist Lawrence Krauss. In addressing the bewildering nature of cosmological questions and answers, he wrote:

"[But] no-one ever said that the universe is guided by what we, in our petty myopic corners of space and time, might have originally thought was sensible. It certainly seems sensible to imagine that a priori, matter cannot spontaneously arise from 'empty' space, so that something, in this sense, cannot arise from nothing. But when we allow for the dynamics of gravity and quantum mechanics, we find that this commonsense notion is no longer true. This is the beauty of science, and it should not be threatening. Science simply forces us to revise what is sensible to accommodate the universe, rather then vice versa."

I find this notion to be rather titillating, as it reveals the honest nature of true science - wherein extraordinary answers can be found if you follow the evidence and entertain the conclusions :).
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