Still Lost in Space

1
Still Lost in Space

• Assignment 6
• Model answer to Assignment Five plus the new data
  youve been asking for.

2
Conclusions from the first Data Release

• What could you conclude from the first set of
  data?
• This universe looks superficially like our own,
  but on closer examination is actually quite
  different.
• Perhaps the biggest surprise was the enormous
  parallaxes of the stars.
• Taken at face value, this would imply that they
  are only a few AU away! A few are almost as close
  to us as the Sun is to the Earth.

3
Really Close?

• If they really were this close, they could not
  possibly be stars. As they appear as bright as
  stars but are 104 times closer, by the inverse
  square law, they would have to be 108 times
  fainter. No such stars exist in our universe
• Also any star that was so close (down to 1.5 AU)
  would be clearly visible as a disk, even with the
  naked eye, let alone a telescope.

4
Two Possibilities

• So we are left with two possibilities
• 1 They are not stars, but some different and
  weird type of object.
• 2 They are stars, but the universe has a
  strongly saddle-shaped geometry that warps the
  angles and causes our parallax measurements to
  give spuriously high values.
• Which is true?

5
If they are stars?

• If they are stars (and the universe is very
  curved), then they should have stellar spectra.
  They do not.
• Also, only a few of them should pulse, and any
  pulsations should take hours to months.
• Instead they all pulse, some with periods as
  short as a quarter of a second! No stars do this.
• Could the pulsation be due to something orbiting
  each of them very quickly? Even if the orbiting
  thing was travelling at the speed of light (very
  unlikely) it could only travel less than 75,000
  km in 0.25 seconds, which means the radius of its
  orbit must be less than (probably MUCH less than)
  12,000 km - which is smaller than virtually any
  type of star.

6
Oscillating Universe?

• Is there any way to save the star hypothesis?
• The pulsations could be caused by an extremely
  rapidly oscillating universe.
• But then all stars should have the same period,
  which they do not.
• Perhaps different bits of the universe are
  pulsing at different rates?
• General Relativity says this could only happen if
  the mass distribution is also pulsing by vast
  amounts on comparable timescales, which seems
  unlikely.
• And anyway, this wouldnt explain the weird
  spectra.

7
Not Stars

• So the simplest conclusion is that these things
  are not stars, or at least not stars as we know
  them.
• They are something strange and different.
• We thus no longer need the saddle shaped
  universe hypothesis. Which is not to say that it
  isnt true - just that we have no evidence for
  it.
• Lets tentatively assume (as a starting point)
  that the geometry of space is not radically
  curved.
• Then these stars really are far fainter than
  real stars, and really are only a few AU away
  from us.

8
What are these Stars?

• What then can we learn about these stars that
  arent stars.
• They must be pretty small, otherwise wed see
  disks, given their close distance. Much smaller
  even than planets (Mars and Jupiter are both
  further from the Earth than many of these things,
  but easily show disks with even small
  telescopes).
• The data show some striking patterns.
• The pulse period, wavelength and pulse amplitude
  are all strongly correlated.

9
All the same?

• Could all these stars be basically the same
  type of object, only lying at different
  distances?
• Perhaps they are extremely compact emission-line
  nebulae. Normally any nebula would emit multiple
  lines, but perhaps these nebulae contain only one
  element, and very unusual excitation mechanisms,
  so that only one line is produced?
• In this case, the different wavelengths would be
  explained by an expanding or contracting
  universe. The expansion or contraction rate would
  have to be enormous to give such big
  red/blueshifts for objects so close.
• Unfortunately there is no correlation between
  wavelength and distance.

10
Lumpy Matter

• It is, I suppose, just possible that different
  bits of the universe are expanding and
  contracting, and thus explaining different
  redshifts for stars at the same distance.
• But even that would not explain why the amplitude
  of the pulsations correlates with the wavelength.
• And it would require a very strange distribution
  of vast moving quantities of dark matter on very
  small length scales (remember, the metric isnt
  arbitrary - it is controlled by the distribution
  of matter).

11
Doppler Effect?

• Could the different wavelengths be caused by the
  Doppler effect - different objects moving at
  different speeds?
• That wouldt explain the correlation between
  pulse amplitude and wavelength.
• And the necessary speeds would be a good fraction
  of the speed of light.
• Given that the objects are only a few AU away, we
  should easily see them moving at these huge
  speeds, and we do not.

12
Different Objects

• So the most plausible explanation is that the
  objects emitting at different wavelengths really
  are different types of object, and not just the
  same class of object seen at different redshifts.
• So what are they? Could they all be compact
  nebulae, but each with a different element and
  hence a different wavelength?
• Youd need a whole lot of elements, it would
  still be puzzling that you only see one line, and
  the correlations with pulse period or amplitude
  would not be explained.

13
One family

• It rather seems as if we are looking at one
  family of object, with somewhat variable
  properties.
• The more distant ones appear fainter.
• If you use the inverse square law to compute the
  real luminosities of these objects, they all come
  out roughly the same.
• Furthermore, this luminosity correlates with the
  frequency - redder objects are brighter.

14
Like Nothing Weve Seen Before

• So - these stars are like nothing weve ever
  seen in our own universe.
• They are extremely small, much fainter than
  stars, and emit all their light at one
  wavelength.
• Their luminosity, wavelength, pulse period and
  pulse amplitude all correlate nicely together,
  suggesting that they are one family of object.
• So what are they? Nobody has any idea.
  Suggestions being bandied around are white holes,
  worm holes, strange neutron stars and much more.
• Clearly more data are needed.

15
More Data

• You have now been stranded through the wormhole
  for a day.
• Youve put together some new equipment and made
  lots of new observations, guided by the initial
  data.
• A bigger telescope was built, and pointed at the
  nearest of the stars, only 1.5 AU away, if you
  believe the parallax data.
• Even with a resolution of 0.03 arcseconds, it
  looked like a dot.
• Youve been tracking several of the nearer stars,
  looking for signs of motion. No transverse motion
  was seen - the angle to each star from your ship
  remains constant.

16
Triangle!

• In an attempt to measure the geometry of space,
  you sent out a second probe, at right-angles to
  you and the first probe.
• Laser beams were exchanged between both probes
  and you, forming an equilateral triangle, 10,000
  km on a side.
• The interior angles of this triangle added up to
  179.99996 degrees, with an error estimate of
  ?0.00005 degrees.

17
Microwave Background

• No microwave background was detected. Any
  emission warmed than 1K would have been pixed up
  with your equipment.
• A puzzle - measurements from the probes indicate
  that the vacuum outside the ship is much emptier
  than even typical intergalactic space in our
  universe.
• They are picking up no atoms at all that hadnt
  leaked out from the USS Drongo or the probes.
• Also, no cosmic rays are being detected.

18
Wider Wavelength Coverage

• Youve managed to put together a spectrograph
  that can observe UV light in the wavelength range
  10-400nm, and IR light in the range 800-2500 nm.
• You pointed this spectrograph at dozens of stars
  and saw nothing in any of them. They do not
  appear to emit any radiation in these bands.

19
Radio Array

• While youve continued with your observations,
  the sensor division have been rigging up some
  pretty nice equipment.
• Theyve built a radio array that allows them to
  pinpoint exactly where in the sky the mysterious
  radio bursts are coming from.
• They then provide you with the coordinates, and
  you then point your big new telescope in that
  direction.

20
Imaging the Bursts

• Remarkably, each burst appears to be coming from
  a star.
• Most of these stars are much fainter than the
  ones youve been studying so far - thats why you
  needed the bigger telescope.
• The stars appear quite normal. They have spectra
  much like the other stars, and they too pulse in
  brightness.

21
Different Colours

• These radio emitting stars have spectra peaking
  at a wide range of wavelengths, though they
  perhaps are more concentrated at longer
  wavelengths.
• Indeed, several were not initially detected at
  optical wavelengths, and were only found when you
  hooked up your infra-red spectrograph to your new
  telescope.

22
Data Table

• Once again, Ive provided an Excel file
  containing the data on these sources.
• It includes both the radio data on the bursts and
  the subsequent optical data.
• You tried to measure parallaxes for these
  sources, using the small space-probe exactly as
  before.

23
Assignment

• The details of this assignment (ie. word limit,
  group work etc, how to submit) are identical to
  the last one.
• The deadline is 10am on Thursday 12th June.
• Once again, you should try and deduce as much as
  you can about the strange universe in which you
  find yourselves.
• Top priority is determining the large scale
  cosmology of this universe, as this is what will
  help you generate a wormhole to get you back home.