Top Achievements for the NRAO Very Large Array

Question:  I’m embarrassed to ask this, as I feel after working here for so long I should already know the answer. But here it is: What are the top 3 discoveries/achievements of the VLA? And why?  — Doug

Answer:  As you can imagine, an actually ranking of the Very Large Array (VLA) achievements would be open to debate.  Kind of “beauty in the eye of the beholder” problem.  Dave Finley, NRAO Public Information Officer, provided the following links listing what NRAO believes are the VLA’s top scientific achievements:

Jeff Mangum

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Dark Energy, Universal Expansion, and Peculiar Galaxy Motions

Question:  I have been hearing lately in some space documentaries that black energy, which makes the universe expand and at an increasing speed, separate and repulse objects such as galaxies from one another, am I correct?  But i also heard that the Andromeda galaxy is heading straight for us and should merge with the milky way, isn’t it contradictory? Thank you in advance for taking the time. – David

Answer:  I suspect that what you mean by “black energy” is really Dark Energy, which is one of several explanations for the acceleration of the expansion of the Universe.  The folks at NASA have a very nice page which describes what we think Dark Energy is, which you should certainly check out.  You are correct that Dark Energy is theorized to act like a repulsive force which pushes galaxies apart.  As for the Andromeda galaxy’s peculiar motion toward us, this is due to the mutual gravitational pull between our galaxy and Andromeda.  This is a “small-scale” affect, while the overall expansion of the Universe happens on a larger scale.

Jeff Mangum


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Will Dark Energy Tear Apart Black Holes?

Question:  Will dark energy eventually tear apart black holes, especially supermassive black holes?  – Reinaldo

Answer:  Let’s start with a basic understanding what what Dark Energy is thought to be.  It is a hypothetical form of energy which is thought to pervade the universe.  It has been proposed as the source of the measured acceleration of the expansion of our universe.  As a physical entity it is really quite “thin”, having a density of about 10^(-29) grams per cubic centimeter, so its affect of things like planets, stars, humans, and black holes is exceedingly small.  But, because it is thought to fill the universe its affect on the expansion of the universe is very important.  So, to answer your question, Dark Energy does not affect the properties of black holes, so in fact cannot disrupt them in any way.

Jeff Mangum

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What is the Difference Between the Redness of an Object and its Redshift?

Question:  What is the difference between Doppler shift of light and Natural light spectra (blue for young and red for old) with respect to stars and galaxies? How does astronomers differentiate between Doppler shift and Natural light spectra of deep space objects?

For Example: Consider a deep space object which is actually young (blue color) and also receding away from earth but due to its enormous distance from earth it is red-shifted (appears as red-colored in Hubble observations) due to Doppler effect. In the above condition, how does an astronomer determine the true color of the cosmic body?  – Vinod

Answer:  The color of an object is determined by the peak in its continuum emission, which is itself determined by how hot the object is.  For example, a star that appears red is colder than a star that appears blue.  Redshift, on the other hand, is a property determined by the shift in wavelength of spectral lines emitted by the gas in an object.  Now, a redshifted object will also have a redshifted peak to its continuum emission, so it also will appear redder.  If we can measure the redshift of this object we can determine its true color by correcting for the reddening caused by high recession velocity.

Jeff Mangum

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What is the Flux Density of the Cosmic Microwave Background?

Question:  The temperature of the CMB is about 3 Kelvin, but the 3 K here mean the brightness temperature or the antennna temperature ? what is the CMB flux density ? – Hongmin

Answer:  The CMB temperature is the physical temperature that a black body would have in order to emit the radiant flux we measure.  The flux density of the CMB at a given wavelength can be calculated using this fact that it emits as a black body with a temperature of about 3 Kelvin.  That leads to a peak flux density of about 380 million Janskys per steradian at a wavelength of 2mm.

Jeff Mangum

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Radial Extent of Last Scattering Shell at CMB Recombination

Question:  At CMB recombination (z=1090), what is the radial extent of the last scattering “shell”?

  • a) Delta(z) = ….
  • b) Delta(comoving angular distance)= ….Mpc

The WMAP first-year parameters give Delta(z) = 195. Is this still correct?  – Rene

Answer:  I think that the Planck results did not change the standard cosmological parameters very much.  Therefore, the WMAP value you quote should still be the correct value.

Jeff Mangum

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How Did the First Galaxies Form?

Question:  From where did this initial cloud of gases and dust come? In other words, how did the cloud form? Why were these gases and this dust together in a cloud?
I read on another website that the cloud began to collapse because it was rotating. You say that the cloud began to rotate as it collapsed. I assume the discrepancy is because of either your or the other writer’s uncertainty. As the cloud collapsed, why would it have begun to rotate? What would have caused the cloud to collapse in your scenario?  – John Paul

Answer:  I presume that you are referring to the formation of galaxies.  Starting from the beginning, the first galaxies are believed to have formed as a result of primordial fluctuations in the early universe that gravitationally attracted gas and dark matter to the denser areas.  These structures became the first galaxies.  These first galaxies began to collapse due to self gravity, with rotation being part of this process in order to dissipate energy via angular momentum.  Remember too that that this early stage the universe was almost exclusively composed of hydrogen, helium, and dark matter.  Soon after these first galaxies formed, the hydrogen and helium gas within them formed the first stars, many of which were quite massive, evolved quickly, and ended their short lives as supernovae.  The supernova process expels processed matter into the interstellar medium of the young galaxies, matter that contains heavier elements (produced by the fusion process in those first young stars) and some dust.  That process of gravitational collapse, stellar evolution, and enrichment of the interstellar medium in galaxies then continues until we get the universe filled with galaxies we see today.

Jeff Mangum

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Time Dilation and the Speed of Light

Question:  T or F: Since gravitational time dilation has been confirmed to exist. The speed of light cannot be constant since speed = distance÷ time and if the denominator changes then the speed must also change? Or atleast the distance would have to change to keep the equation in balance.   – Mike

Answer:  Time dilation is the difference of elapsed time between two events as measured by observers who are located in different gravitational environments.  The speed of light in any one of these locations is always equal to c.  A time delay is measured by a remote observer who monitors the passage of light near a source of gravity, such as our Sun. You can think of this as causing the path of the light to be “curved” near the gravitational object, thus lengthening the distance, and time, needed for the light to reach the remote observer.

Jeff Mangum


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Cosmic Inflation

Question:  Has anyone considered that the increase rate of expansion of the universe may be do to a combination of the universe’s rigid body mode (Big bang expansion) and flexible body mode or modes of vibration of the universe caused by the big bang (matter/anti-matter explosion), and not resulting from dark energy?

This would produce positive acceleration rate, at times, for the universe, which would not expected by rigid body acceleration alone.

I based my assumption of excitation of the universe’s natural frequencies occurred right after the big bang (matter/anti-matter explosion), when matter first existed. At this time the universe was, based on internet research, 1000 times the diameter of our solar system (5,913,520,000,000 km) and the mass presently is 1.59486e+055 kg. This would give an average density of 665.167 lb/in^3, or about 2,350 times the density of steel. For me, this would be an excellent environment for the excitation of the natural frequencies of the universe.

The natural frequency excitations would also produce nodal lines of concentrated mass, as seen in the universe today.  – Kari

Answer: In fact, there is no resonant frequency associated with the universe.  This isn’t a physically meaningful quantity to associate with the universe or the big bang.  I believe what you are looking for is an explanation for the quantum fluctuations within cosmic inflation which resulted in the current structure of our universe.  The link I have provided is a nice detailed description of cosmic inflation which should provide a good description of the current theory.

Jeff Mangum

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Relativity and the Twin Paradox in a Spherical Universe

Question:  Would relativity hold true if the universe had spherical geometry? For example, the famous twin paradox is resolved because the twin that leaves the Earth has to turn around (accelerate) to get back home. If the universe was spherical, then he could just travel in a straight line and come back to the Earth without ever having to turn around.  – Christina

Answer:  As you might expect, the twin paradox has been studied for the general global shape of space, whether it be cylindrical, toroidal, or spherical.  The paradox enters when in a spherical dimensioned space when the travelling twin remains in an inertial frame, never accelerating.  Since both twins are in inertial reference frames, they suffer the same time dilation, but both view the other as having suffered time dilation.  Calculations show, though, that the travelling twin is actually younger due to the asymmetry in the inertial frames of the twins.  The answer lies in the fact that these curved geometries possess multiply-connected topologies, which makes breaks the symmetry of the inertial reference frames for the two observers, which resolves the paradox.  If you are interested in a very detailed physics-based explanation of the twin paradox, see Jean-Pierre Luminet’s nice overview.

Jeff Mangum

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