universe - What happens to photons that don't interact with something?

Say a photon left one of the first stars ever created in our universe. This photon has been traveling for 13+billion years through the vastness of space without ever hitting anything. Sure, it's probably been tugged at by gravitational interactions, but it's still happily traveling at the speed of light without direct interaction with matter.

As we know, most of the universe is composed of mostly empty space. The odds of any single photon hitting something (like the Earth as starlight, etc) would seem to be very small.

What will eventually happen to this photon? Will it eventually reach the other end of the universe, and if so, what would happen to it there?

interstellar travel - Are there any planned extra-solar missions by NASA or others?

As David Hammen stated in his previous answer, neither NASA, ESA or any other space program has any extra-solar mission in the works. However, there are many studies of interstellar and interstellar precursor missions, some of them ongoing.

Perhaps the best known and most advanced interstellar probe is Project Icarus (wiki). This is a modern redesign of an earlier project from the 70s known as Project Daedelus. Icarus Interstellar is a collaborative project of the Tau Zero Foundation and the British Interplanetary Society, the latter of which came up with Daedelus. Both ship designs rely on nuclear propulsion to attain an appreciable fraction of the speed of light in order to reach a close star system within a century.

NASA produced a study on what they called the Innovative Interstellar Explorer:

This study focused on the elusive quest to reach and measure the interstellar medium, the region outside the influence of the nearest star, the Sun. It proposes to use a radioisotope thermal generator with ion thrusters.(1) The project is a study of a proposed interstellar precursor mission that would probe the nearby interstellar medium and measure the properties of magnetic fields and cosmic rays and their effects on a craft leaving the Solar System.(2)

Using an ion drive with xenon propellant, it could attain 1000 AU within 100 years of launch (updates here and here, slide show here).

Another interstellar precursor probe is Project FOCAL, a proposal by Italian physicist Claudio Maccone. The idea is to send a probe out to 550 AU and beyond in order to use the Sun as a gravitational lens.

I would be remiss if I did not mention the 100 Year Starship Project, headed by American physician and former NASA astronaut Mae Jemison. This is a DARPA sponsored project to come up with a business plan for research on interstellar travel over the next hundred years.

solar system - What implications do younger Earth and Moon have on Late Heavy Bombardment genesis hypotheses?

The Late Heavy Bombardment (LHB) or the Lunar Cataclysm is when the inner solar system, including the Earth and the Moon, underwent multiple and sustained heavy impacts early in their history (around 3.7-4 billion years ago). This event formed many of the major impact scars we see on the Moon, (presumably as well as much of the impacts observed on Mars and Mercury). Two main theories are based on perturbations in either the Kuiper Belt, or in a primordial larger asteroid belt.

What implications would a younger Earth and Moon, discussed in "Moon and Earth may be younger than originally thought" (Lars Borg et al., 2011) have on hypotheses on what caused the LHB?

space - How and when did we learn about the universe?

You're asking a very big question - basically, "what is the history of astronomy?" It's a very long story. Our knowledge about the size of the Universe grew slowly, bit by bit.

The ancient Greeks knew that Earth is round, like a sphere, and measured its size with surprisingly good precision.

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

In the 1600s, with the invention of the telescope, and the idea that the Earth revolves around the Sun, scientists (Johannes Kepler and others) figured out how measure the distance from Earth to Sun. After that, it was easy to measure distances within the Solar System.

http://www.tbp.org/pubs/Features/Su04Bell.pdf

In the 1800s, we made the next step: measure distances to other stars nearby. This is something called parallax.

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

In the 1910s, we figured out how to measure not just the distance to nearby stars, but the whole size of our galaxy. This was done by measuring variable stars (stars that go brighter, then dimmer, then brighter again, and so on).

http://www.aip.org/history/cosmology/ideas/island.htm

After that, in the 1920s, Hubble figured how to measure the distance to other galaxies - that has been answered already on this page.

http://en.wikipedia.org/wiki/Edwin_Hubble#The_universe_goes_beyond_the_Milky_Way_galaxy

---

Different methods are used to measure different distances. Each method measures up to a certain distance; after that, a different method must be used. This is called the Cosmic Distance Ladder.

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

In any case, there's a lot to talk about on this topic. You may want to just grab a book on the history of astronomy and read. What you're asking is basically what's the whole history of astronomy, after all. :)

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

Here's a book that, even though pretty big, should be quite accessible to someone your age:

http://www.amazon.com/The-History-Astronomy-Heather-Couper/dp/1554075378/

What effect does the Earth's rotation have on plate tectonics?

I've been reading up on plate tectonics and found that there are several different driving forces involved. One of the main categories of driving force mentioned on Wikipedia is forces related to the Earth's rotation. The section on driving forces related to Earth rotation presents the information as a sort of historical dialogue of the scientific process, which is somewhat helpful but has left me confused as to exactly what role the Earth's rotation does play in plate tectonics.

So how does rotation affect the movement of plates?

fundamental astronomy - Acronyms of catalogs in SIMBAD database

It's roughly like this:

set limit 100
format object form1 "%IDLIST(1) : %OTYPE(S)"
query sample (otypes = '*i*') & (bibcode='2009MNRAS.394.1338B')

To get the list of the bibcodes choose
"Other" / "Catalogue collection"
from the menue bar.
The screen should look like

Then click the icon "Click to display the menu" at the left border:

Here you find the bibcode:

regarding spectral type of a star

The key for stellar classification is the Hertzsprung-Russell diagram (or HRD for brevity), also know as the color-magnitude diagram.

This shows the relation between the surface temperature (on the x-axis) of a star and its luminosity (on the y-axis). The temperature is an index of the Spectral type. For distant stars also spectroscopy is taken into account, to classify spectral types according to their absorption lines here some examples. The usual classification is called MKK (from the astronomers that had developed it - Morgan, Keenan and Kellman in 1943), and divides the stars in 7 stellar types O, B, A, F, G, K, M. Please note that from the 1943 other stellar types have added, but these are the most used. This sequence goes from the hottest (O) to coolest (M) stars. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest. Our Sun is a G2 type star. This classification also uses an additional notation to avoid the "degeneracy" you mentioned: a roman numeral between "I" (one) and "V" (five). The higher the number, the wider the lines in the observed spectrum. However, despite of the quantity these roman numbers indicate, they are an index of luminosity class. This is based on the width of certain absorption lines in the star's spectrum which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class I stars are supergiants, class III regular giants, and class V dwarfs or main-sequence stars, with II for bright giants, IV for sub-giants, and VI for sub-dwarfs (from here). Then, our Sun is "completely" identified by the type G2V. You can take a look at the figure below, and check out by yourself where it belongs:
.
Also note that other indexes are used to characterize the stars, as for the width of the lines in the luminosity class, or for some peculiarities.

Finally, if you are interested in the study of stars, you should always keep in mind the Stefan-Boltzmann law:

$L=4pisigma R^2 T^4$

By this you can identify isoradii lines in the HR diagram:

Also, there is a proportionality between luminosity and mass:

$Lsim M^{3.5}$

Then you can identify "isomasses" lines in the HR diagram, but I can't find such a figure to show.

If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

Gravity is a force, and it need not have a "speed"¹.

A gravitational body sets up a gravitational field around itself. Note that by the time the light wave approaches the body, this field has already been set up. The gravitational force does not need to "reach out" and "catch up to the light" — it's already there.

In fact, it's not even really a force (though we can treat it as one to an approximation). Gravity bends the fabric of spacetime around it, messing with the meaning of a "straight line". From different reference frames, different lines appear straight. To the light wave, the inspiraling path seems "straight" and thus it follows that path. To an external observer, the light wave is not going straight.

^{1. It does, but that's the speed at which changes to the field propagate, not the speed at which it "catches up" with other things. Changes in the gravitational field propagate at lightspeed.}

solar wind - Does the Moon have an aurora?

Technically, yes, the moon does have an atmosphere that gets ionized by the sun.
NASA: Is There an Atmosphere on the Moon?

our moon does indeed have an atmosphere consisting of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars or Venus. It's an infinitesimal amount of air when compared to Earth's atmosphere. At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume.

Pretty thin soup. Sodium and potassium get kicked up by photons hitting the surface. There's also argon, helium and other minor components.
As shown in this figure from the first link: The sodium in that atmosphere does glow; (Rayleigh unit definition). At least some of the glow is due to full fledged ions rather than charge-neutral fluorescence:

It is confirmed that the ions include oxygen, sodium and potassium.

So, whether by ionic or fluorescent emission, the moon's atmosphere does glow. Unless one wants to argue about the glow not being restricted to the poles by a magnetic field, or airglow, that glow counts as an aurora.

Matter/mass exist into the space. Where does space exist?

There is still no way to solve this question for all people.
You also need to keep in mind that time is directly linked to space. So before the universe there was no space and no time. However, if you want describe processes or even states you should also be able to talk about time.
Time is the perception of a sequence of events. So before the big bang no events could happen. To be more detailed time is what physicsists call the increase of entropy. Entropy itself is the randomness of a closed physical system. Each physical systems wants to have maximum entropy or randomness. The big bang at point of time -zero- can be seen as a point with the lowest possible value of entropy, because any matter/energy was located at a single point - at a singularity. There was ne place for particles to get at a random place.

With the expansion of this point space and time expanded into "nothing". If you think about it: how could it expand if there was something?

Next: If there is something outside, we still have a neverending question: What's outside the outside? Is it infinite?
Our physical laws don't work outside - so how should we talk about things nobody could ever describe?

Of course, there are theories that try to explain why or how the big bang happened - the ethereal expansion by Alan H. Guth and Alexander Vilenkin, for example. Universes are "created" like holes in a cheese by massive discharges of energy (like lightnings during a storm), but those are very hypothetical theories without any prove or even evidence. And all those theories need parallel universes / multiverses.

So, for my opinion, I keep at this point: There was nothing. As human beings we cannot deal with nothing. For us there is always something. If we try to think about nothing, we still think on how much time has passed not thinking about anything - for example. Nothing is mathematically described as 0. But also 0 is a number. It's more than nothing. It still consumes place in a formula. So don't think about nothing as "nothing". Think about nothing as everything. Because or universe was created out of nothing, this nothing is also everything for any or every time.

How would an exoplanet be found from earth if our view of its star system is "from the top"?

First of all, it is not that you can always find the planet (if it is there).

Anyway, if this is the case, when we observe the system not totally face-on, we resort to radial velocity measurements. The principle is simple, the actual much less.
The orbiting of the planet around the star causes the star to "oscillate" around the center of mass.
Here you can see a good example:

However, when the system is exactly face-on, the Doppler shift is useless, but still the gravitational effect of the planet onto the star is useful and used.
Here is what happens in the latter case:

It is clear that the star undergoes a change in position.
Then usually, the radial measurements go along with astrometrical measurements, to identify the position of the star along the planet's orbit.

Of course, you need some conditions are verified to allow this measurement.
First of all, you planet must be massive enough to bring a sensible change in the position of the star. Then your instrument must be sensible enough to resolve the tiny effect originated by this change.
Possibly, also other conditions play a role, but I am not an expert (you can take a look at here).

For a general comprehension, also a wiki for Methods of detecting exoplanets.

formation - What is in the center of the universe?

I have pondered this issue for nearly 35 years. If the universe came into being by way of a Big Bang process we will probably never find the actual center at which it commenced:

REASONS A CENTER MAY NEVER BE FOUND

1. First, we should always keep this in mind: We did not see the beginning of our universe take place. There are no eyewitnesses who can tell us what actually happened. Therefore, it is a 13.7+ billions of years old COLD case. In other words everything we put forward regarding the beginning of the universe will always be only speculation.

We have no way to prove anything that we hypothesize regarding how our universe commenced (no matter how many theories are proposed regarding the beginning of the universe nor how good they and their corresponding math equations appear to be, there is no way to fully test them to prove out anything they make claims for).

In other words, even if the numbers do not add up perfectly you can always come up with another constant or sub-theory that makes it look more correct, none of which is provable in our real world predicament. This means it may never be possible to determine if a BB took place or if a center ever existed at all.

2. That said, even if the Big Bang is responsible for how the universe expanded (a kind of explosion of a plasma type soupy mix under pressure against gravity) to its current state, problems or barriers may prevent us from locating the real location of the center from which it all supposedly started.

3. The starting point, regarding the BB, was supposedly a state of singularity. What singularity actually would be is not really known. However, its state would be a point about the size of a marble in which presumably all the laws of physics as we know them are broken down, a kind of state of total annihilation of all atomic structure and its various particles. If the supposed early "super" inflation phase occurred it would be opaque because photons would not be present nor released during this phase. Therefore, our view of events up to this point would be obscured.

In other words, we can never see through this phase nor would there be any way to see this phase of inflation because we would need to see photons. Only where photons are released are we able to see even part of what was there (the so-called visible universe limit).

4. Because we can not see beyond the visible universe (which would include the parts of the electromagnetic spectrum that can not be seen by the naked eye, that can only be seen through equipment that can detect invisible wavelengths) we would have almost no chance of ever finding the real center of the BB.

5. According to one theory space exists and inflates separate from matter and energy. That is we are located within and are a part of this inflation. According to this proposal space (fabric of spacetime) is inflating in all directions all around us, that we are within the spot, the center. The result being that we can not detect a direction in which the BB started from. Supposedly, because we are located within the BB inflation, this precludes us from having a frame of reference that would allow us to have the ability to trace back and locate the real center or even the general location of the BB starting point.

6. The real world frame of reference for our universe is three dimensional plus time. The visible universe matter is not uniform in all directions. That is all the galaxies, etc. are not uniformly spaced along its x, y & z axis (unlike the two dimensional demonstration models with evenly spaced matter). Since, supposedly the Big Bang started with a very tiny singularity blob (at least for this theory), smaller than a marble, then there ought to be various vector trails (even if they are fragmented or somewhat askew, blurred or dissipated) of some sort going back to the marble size singularity state or at least into the outward edge area of the 'super' iflation phase (especially since matter is not uniformly spaced and would have to change direction and consistancy in order for large areas or gaps at irregular intervals to be present as they are now).

Even if no hole or hollow area exists there should be a huge clouded area or an area with different characteristics than most of the rest of the universe if a BB took place. However, so far we have not been able to locate any evidence that would suggest that the BB came from a particular direction which has also precluded us from locating a BB center.

7. Unfortunately, two dimensional demonstation models lack ability to show true three dimensional effect regarding starting at a tiny point through the present state of inflation or expansion of 13.7 billion light years. In other words, if we look across through our universe sphere (matter does not exist evenly spaced on two dimensional planes here) somewhere at least near part of the edge of our visible universe we should see at least some large area where galaxies are much closer together compared to say an eight or ten billion light years area around where our galaxy is located now. So far we have not found any evidence. Perhaps this means the BB did not occur.

Certainly if the universe had a tiny starting point there should be a visible change somewhere in an inflation or expansion that covers such a huge area of more than 13.7 billion light years (there would be some indication in which direction we should concentrate our effort to find something regarding at least the general direction of the BB's original location). So far such indication has not been found.

8. Also, there may be a force(s) outside our universe that is the driving force for the inflation or expansion of our universe, in which case it would most likely make it impossible to ever find evidence of a center if one ever existed.

9. If any kind of BB happened at all I seriously doubt that the universe could be 'flat' unless there was an amazing powerful set of unkown events that caused the universe to change shape. Generally a roundish center that inflated with tremendous outward force would retain a general roundish shape even for our current universe. Perhaps if unseen forces from outside the universe caused some huge obstruction to the shaping process that would have come by way of a BB, there could be a different shape now. However, I doubt that there will ever be any way to varify such a claim.

rotation - Faster Earth spinning speed to overcome gravity, possible?

As far as I know we stick to the surface of the Earth as its gravity is pulling us and Earth rotation is in a way reducing the gravity.

At what speed should Earth be spinning so that we all get ejected / catapulted into space (if we all line up on the equator).

Sub question: at what speed of spinning would we all “stick” a few meters above the ground?

Why there are other planets in our solar system?

Did you mean to ask

Would life on Earth be possible if Earth was the only planet in the Solar system?

The answer is perhaps , we can only speculate and know too little about the evolution of the planetary system to give any firm answer.

The Moon, for example, (not a planet I know) stabilises the Earth spin axis, preventing it from flipping, which would otherwise happen (on long time scales) with drastic consequences for the Earth climate and hence any life forms.

Jupiter (and to a much lesser degree the other outer planets) plays a significant role in the dynamics of smaller bodies (asteroids and meteroids), including absorbing them. If, for example, the meteroid and asteroid impacts on Earth were more frequent, that too would have drastic consequences for the climate and life.

cosmology - What happens to the shrinking universe in the presence of the pressure of Hawking radiation?

In a shrinking, roughly 3-spherical universe with only a black hole, Hawking radiation should follow a geodesic line and return to the black hole, without excerting radiation pressure to the universe as a whole.

Therefore it's hard to see, how Hawking radiation should establish an equilibrium with gravity.

More feasible seems, that the shrinking universe can prevent the black hole from further evaporation at some point, since evaporated particles and radiation are directed back into the black hole by the overall curvature of spacetime.

spectroscopy - How to specify SM libraries for MOOG

I am running MOOG on OS X Yosemite and with $MOOG running the abfind driver I can force fit elemental abundances with an appropriate input line list and model atmosphere. MOOG crashes, however, when I try to use it to generate plots, even though I know I have SuperMONGO (sm) installed on my computer because $sm works fine. In particular, MOOG will crash with the following errors:

*******************************************************************************
                  MOOG IS CONTROLLED BY DRIVER abfind                      
*******************************************************************************

wav. correl.:  slope =  -7.791E-07  intercept =   7.505  corr. coeff. =  -0.122
Fonts file not found
File graphcap is not defined
Can't get graphcap entry for nodevice
No such device nodevice
File graphcap is not defined
Can't get graphcap entry for x11
No such device x11 -bg black -title MOOGplot -geom 700x800+650+000

How can I get MOOG to find these libraries?

planet - Does tidal heating imply orbit degradation?

There is a wonderful post at Physics Stackexchange:

Gravitational coupling between the Moon and the tidal bulge nearest the Moon acts as a torque on the Earth's rotation, draining angular momentum and rotational kinetic energy from the Earth's spin. In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period. As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down.

You can find more reading material and information in the original thread.

astrophysics - Calculating the mass of star

You don't say what other information you have for the "several stars". Yes, you can use a mass-luminosity relationship if the stars are on the main sequence. In terms of mass uncertainties I would estimate that you might be at the level of 20% unless you can absolutely pinpoint them on a Hertzsprung-Russell diagram, because the luminosity of a (fixed mass) main sequence star changes, even whilst it is on the main sequence.

If the stars are in any other phase of their evolution then your problem is much harder, even if you can place them on the HR diagram. Not least because there is considerable disagreement between different evolutionary models as regards the positions of stars of a given mass at a given age, on both the pre- and post-main sequence. Furthermore, their luminosity and temperature evolution can be influenced by factors such as rotation, metallicity, magnetic fields and mass-loss.

Compact stellar remnants and brown dwarfs do not follow a mass-luminosity relation. In general you can only estimate their masses if they are in binary systems, or for white dwarfs, you can estimate their radii and either their surface gravity or gravitational redshift. Estimating a brown dwarf's mass needs both its luminosity and age.

tidal forces - Are many exoplanets synchronously tidally locked like Mercury?

The only tidally locked planet in the Solar system i Mercury. But it is synchronously tidally locked 3:2, because of the relatively high eccentricity of its orbit, so doesn't turn the same side towards the Sun. I wonder if this is common for exoplanets?

Many discovered exoplanets are close to their star and must be tidally locked. Has any of them been determined to be synchronously tidally locked, like Mercury? Is Mercury a rare freak in this respect, or a representant of a common phenomenon?

the sun - If we were to see the Sun with our naked eyes from the Orion belt, would all planets be encompassed inside the star? Is this calculable?

Such observations are possible, see Fomalhaut b.
It depends mainly on the diameter of the objective of the telescope (diffraction limit) and contrast. Telescopes can (at least in theory) be combined to an effective telescope of larger diameter (aperture) by interferomentry.
Masks can help to occult bright stars to overcome the contrast limitatios.

There are indirect methods to "see" faint planets, as e.g. applied in the Kepler mission, or by Gaia.

And yes, it is calculable. With naked eye we wouldn't be able to distinguish the sun from the planet orbits from a distance of about 1000 lightyears, as the orion belt stars: 1000 lightyears are about 300 parsec. This means, that the earth orbit would be about 1/300 arc second. The resolution of the human eye is about 4 arc minutes, the 72,000-fold. For Pluto it's still more than 1000-fold.

gravity - Dark Flow: statistical limits on existence

The dipole in the microwave background indicates motion of the Milky Way and thus of the whole Local Group, at least, at about 600 km/s in a certain direction. The straightforward explanation is that the density irregularities nearby from superclusters and voids result in a net gravitational acceleration that, over the age of the universe, resulted in this velocity. A number of superclusters (Hydra-Centaurus, Shapley, and Norma) happen to line up along this direction and reasonably fit the picture. Also there are some big voids in the opposite direction.
There are two basic ways to quantitatively understand what is the cause. One can make measurement of local flows around these superclusters and voids to determine their masses and thus their net effect on the MW velocity. Or one can try to measure the "dipole motion" on larger scales, that is at what size scale does this motion drop to 300 km/s and what scale does it drop to near 0? A number of groups have tried to do this and the results are in disagreement. In particular, one group found no drop off in motion out to fairly large distances. Therefore, an alternate explanation was proposed that the dipole motion is primordial and due to objects beyond the horizon that interacted with us before inflation.

Will we ever settle this? I am sure that we will eventually. There is much work to do measuring the distances of galaxies out to 100 Mpc or so with greater and greater accuracy using new methods (SN Type 1a, surface brightness fluctuations, Tip of the Red Giant Branch) and larger telescopes with better instrumentation and then modeling the peculiar flow on all scales. If the local distribution of matter cannot explain fully the dipole moment in the MWB, then perhaps Dark Flow from before inflation will be necessary to explain it.

Why isn't the dark energy getting decreased?

AS the articles on the web suggest, Dark energy is the reason behind the expansion of universe.

If some articles on the web suggest that, they are mistaken. Dark energy affects the acceleration of the cosmic expansion, but it is not necessary for the universe to be expanding, so it cannot be "the reason behind the expansion".

How the dark energy density changes as the universe expands depends on its current energy density and its equation of state--the relationship between its energy density and pressure. For cosmological constant provides the simplest dark energy model in cosmological models--it's the $Lambda$ in the ΛDCM model--for which the pressure is exactly the negative of its energy density: $p = -rho$. In general relativity, both energy density and pressure gravitate, and in three spatial dimensions expansion the acceleration is proportional to $rho + 3p$ (cf. also Friedmann equations). As a result, a cosmological constant with positive energy density $rho>0$ leads to accelerated expansion, because the contribution of its negative pressure means the overall effect is repulsive.

If so, why isn't it getting used up in doing so.

Somewhat ironically the answer to your question for cosmological constant can be interpreted like so: it doesn't "get used up" because of a kind of energy conservation.

As a given small volume expands, it keeps a constant energy density $rho$ and a constant pressure $p$. Is this situation sensible? Suppose its volume increased by $Delta V$, so its energy content increased by $rhoDelta V$. If the pressure is constant, this is some kind of isobaric process, so the work done is $W = pDelta V$. But if $p = -rho$, then everything is consistent because the dark energy does negative work as it expands to exactly balance the increase in its energy.

In reality, you shouldn't take the above reasoning too literally. There are many issues in interpreting anything like "how much energy" there is in general relativity and whether or not it's conserved, some of which I've covered in this answer, though some of which can be alleviated by the stressing of the "small volume" qualifier. Still, "it doesn't get used up because the equations of the theory say it shouldn't be getting used up", while perfectly true, would not be a very interesting answer.

Additionally, it should be clear from the above that the cosmological constant case of $p = -rho$ is very itself special. The reason for its simplicity is that it's completely Lorentz-invariant: in different local inertial frames, the energy density and pressure of a perfect fluid get intermixed and transformed into different values... except in the case where the pressure is exactly the negative of energy density. Every observer would agree about the cosmological constant, so one can interpret it as an intrinsic energy density of the vacuum. In fact that's how it was originally introduced; other forms of dark energy are a generalization of $Lambda$.

And consequently, the rate of expansion should reduce!

With a slightly more general equation of state $p = wrho$, the cosmological constant corresponds to $w = -1$. Other cases give a perfect fluid, which might indeed "get used up" in the sense of getting more dilute, and for positive energy density, $w<-1/3$ is necessary to outwardly accelerate cosmic expansion. But something curious happens if $w<-1$: we should expect from the work done argument for the energy density to accumulate instead of remaining constant. This kind of "phantom energy" would, if positive, produce a Big Rip scenario, in which the cosmic expansion accelerates to an infinite value in finite cosmological time, ending the universe by tearing everything apart.