Monday, April 04, 2005

Do away with Dark Energy by Space Dusts

Why do we need dark energy to account for critical density of the universe, which is almost 1? It's true that the matter density accounts for only about 30% of needed critical density to obtain a flat universe. But it is also true that scientists have counted only mass of luminent galaxy clusters when they calculate the matter density.

Could it be possible that the huge void between galaxies, contain the other 70%, not in the form of some sort of dark energy, but in some sort of none-luminent space dust, which is just regular baryon matter. Or, the space dust simply do not emit/absorb/reflect any significant number of photons to be detectable by today's technology at all?

Could it be that "dark energy" is simply space dust of regular matters, hidden in the darkness of the universe? That sounds very plausible, consider how low the critical density of the universe is: It averages no more than the mass of 5 protons per cubic meter volume. It is very possible that the density of inter-galaxy space dusts are so diluted, that a photon emitted from 10 billion years away hardly hit a single grain of space dust, before it arrives at earth, but yet it sums up over the whole volume of the universe to account for 70% of the needed critical density?

Let's look at some example we can observe, the micro-meteorites in the vicinity of the solar system. According to this web page:

There are at least 100 tons of material crashing into the earth atmosphere per day. Most of them are micro-meteorites merely a milimeter in diameter. It was also said that all the minerals and metal contents of the earth came from meteorites since the earth's formation.

Let's do an estimate of the density of meteorites per cubic meter of space in the solar system. It can be calculated by divide the mass of meteorites the earth collect in one day, by the volume that the earth swipe through in one day traveling in the orbit around the sun. We know the earth radius is about 6400 km, and it travels about 30 km per second around the sun:

100x10^3 kg / (PI*(6400km)^2 * 86400 seconds * 30km/sec )
= 3x10^-19 kg/m^3

We know the critical density of the universe is only 9x10^-27 kg/m^3, which is only 3x10^-8 of the density of micro meteorites around the earth. Clearly it does NOT take a lot of space dust to account for the 70% missing mass of the universe.

One thing we know is a considerable number of meteorites the earth receives come from outside the solar system. Clearly billions of years circulating around the sun has exhausted most of the space dusts within solar system that can be captured by the earth. It must be the case that such space dusts are being continuously replenished from the void of the universe.

How visible are such space dusts or meteorites around the the earth? What we know is the Hubble Space Telescope can detect big space rocks like hundreds of meters in diameter. But other than that, real small space dusts, like milimeter meteorites, register absolutely no signal, until they become visible crashing down the earth's atmosphere.

There is absolutely no detectability of micro meteorites around earth. And here we are talking about density of space dusts 3.3x10^7 higher than the critical density of the universe, shined on by very intense sun lights, in the vicinity of the earth.

If there is virtually no detectability of space dusts near the earth. What about space dusts that's 3.3x10^7 times more diluted, shined on by star lights that's much dimmer than the sun light at earth distance, and located billions of light years away, instead of in the vicinity of the earth?

You would have to say it's virtually undetectable if such space dusts exist throughout the universe!

Such space dust would not emit much photon to be detected: they are very cold: at equilibrium with the CMB background temperature. Microwave photons are the only thing they may emit. And energy balance would mean they emit as much energy as they absorb.

Could they be detectable by absorbing and shielding certain percentage of star lights from billions of years away, hence making evident their presence? Let's try to calculate the free path distance that an average star light photon would have to travel, before it hits one grain of star light, and be absorbed.

Let's assume the star dust is radius one milimeter grains, and their density is 5 grams per cubic centimeter. That would be 2x10^-5 kg per grain. And each grain of dust provides a cross section of about 3x10^-6 m^2. The mass per grain divided by the critical density of the universe should give the average volume occupied by each grain, which is 2.2x10^21 m^3, which is huge.

2.2x10^21 m^3 volume per grain of space dust, divided by the cross-section of the grain, should give the free path distance of how far a photon can travel before hitting a space dust. My result is 7.4x10^26 meter free path. That is a huge free path distance.

Converting to light years:
7.4x10^26 meter / (3x10^8 m/sec * 31557825 second/year) = 7.8 x10^10 light years

We know the radius of the universe is only 1.4 x10^10 light years. So the free path of star light photons are several times bigger than even the ratius of the universe.

Which means there is only a fractional chance that photons from stars billions of light years away would ever hit a space dust, and be absorbed, before it can arrive at the earth.

No wonder the space of the universe is SO transparent, despite of all the space dust that constitute 70% of the mass of the universe.