Space Pioneers

In April 1959, NASA announced the first seven astronauts.  The Mercury Seven are, in order of birth date:

John Glenn (1921)

Wally Schirra (1923)

Alan Shepard (1923)

Deke Slayton (1924)

Scott Carpenter (1925)

Gus Grissom (1926)

Gordon Cooper (1927)

John Glenn, the oldest of the Project Mercury astronauts and the first American to orbit the Earth, was the last to die, in 2016, at the age of 95.

Gus Grissom (1967) – Apollo 1 launch pad fire
Deke Slayton (1993) – brain tumor
Alan Shepard (1998) – leukemia
Gordon Cooper (2004) – Parkinson’s disease; heart failure
Wally Schirra (2007) – abdominal cancer; heart attack
Scott Carpenter (2013) – complications following a stroke
John Glenn (2016) – complications after heart valve replacement, stroke

Walter Cronkite (1916-2009) and Wally Schirra (1923-2007) covered the Apollo moon missions on CBS—far better than anyone else—and I can still remember the events as if they happened only recently.

Want to know who holds the title for longest duration human spaceflight (so far)?  Valeri Polyakov (1942-) entered space aboard Soyuz TM-18 on January 8, 1994 and stayed aboard the Mir space station until returning to Earth aboard Soyuz TM-20 on March 22, 1995.  That’s nearly 438 days (1.2 years) in space!  Moreover, Polyakov, who is a medical doctor, spent over 240 days in space during his first visit to Mir in 1988-1989, giving a total spaceflight time of nearly 1.9 years.

While Polyakov still holds the record for the single longest duration spaceflight, Gennady Padalka (1958-) has spent more time in space than anyone else: 878.5 days (2.4 years)!

Eclipse Comets

A total solar eclipse, such as that which will be crossing America on 21 Aug 2017, would present a great opportunity to discover a bright comet near the Sun.  Has that ever happened?  The answer is yes.

A comet, perhaps magnitude -4 or brighter, was spotted about 1.4° SW of the Sun during the total solar eclipse of 1 Nov 1948.  The editors of Sky & Telescope write in the January 1949 issue, “British Astronomical Association Circular No. 303, dated November 10, 1948, under the title, ‘The Eclipse Comet, 1948 I,’ reads in part:

There can be little doubt that the bright comet now reported seen in the southern morning sky is identical with the one seen during the eclipse of November 1.  The Times of November 2 in the report of the eclipse from its correspondent at Nairobi stated that a bright comet, with a long tail, was seen both by the crew of an R.A.F. aircraft and by observers on the ground.  The head, it was stated by one amateur astronomer, was still visible a few seconds after the Sun began to emerge.

A cable received by Dr. R. d’E. Atkinson, leading the Royal Observatory expedition, reports photographic confirmation of it, saying it was 93′ from the centre of the Sun in position angle 226°, and was very bright, with a tail.

“Harvard Announcement Card 956, dated November 22nd, reads in part:

Dr. Leland E. Cunningham, Students’ Observatory, University of California, Berkeley, writes: ‘New elements have been determined for the bright comet . . . .  These place the comet in position angle 228° and 104′ distant from the sun at the time of the total solar eclipse on November 1, which are in moderate agreement with Atkinson’s observed values of 226° and 93′, respectively.’

“Thus, although Comet 1948 I was missed by northern observers before it passed perihelion late in October, when its tail must have extended into the evening sky after sunset, the total eclipse of the sun provided a favorable opportunity to observe the comet practically a week before southern observers viewed it in their morning sky.  It well can be called the ‘eclipse comet’ of 1948.”

The editors of Sky & Telescope write in the March 1949 issue, “From The Observatory of December, 1948, we quote part of the proceedings of the meeting of the Royal Astronomical Society held on November 12th, at which Dr. R. d.’E Atkinson told something of his recent eclipse expedition to East Africa, and the discovery of the comet during the eclipse.  Dr. Atkinson said:

I propose to speak mainly about the comet which was observed during the eclipse; as far as our own eclipse observations are concerned, I believe they were successful, but the films have not yet been developed.

The comet, though very bright, was not visible at Mombasa, where we were (98% totality), but several newspaper reports from further north referred to it; they did not sound very convincing.  A photograph was published, but as printed it did not actually show the comet; the accompanying description was also based on an error, as I later learnt.  On the journey to Nairobi, sixty hours after the eclipse, I spoke to an eyewitness, whose account disagreed with that in the paper.  It was not until I had seen the photographs taken by the R.A.F. at Nairobi, and had found that they agreed with eye-witness reports at both places, that I realised it must have been a comet; I then made a very rough measurement of its place on the R.A.F. film, and telegraphed Dr. Merton.  As a result of my interest in these photographs, which were taken at 13,000 feet just within and just outside the shadow, the Air Commodore very kindly let me bring the films home for thorough examination.  [On one picture] very much enlarged from a hand-camera snapshot also taken by a member of the crew . . . the tail is clearly visible; visual observers all agreed that it extended downwards until it reached either clouds or the horizon, and it must have been twenty degrees long at least.  The visible part of it does not point away from the Sun at all; any portion which does this must have been extremely foreshortened.  [On another picture] the scale is larger and the definition much better, but the tail is too much underexposed to show except with a magnifying glass.  Viewed in this way, and accepting the idea that the root of the tail will point away from the Sun, one can see enough indications of curvature to make it seem that it is convex to the west; I therefore concluded in my cable a guess that the motion would be westwards, and this has proved correct.  The comet must certainly have been very bright; these pictures were taken with an aperture of f/5.6 and an exposure of 1/300 second; moreover, the head was visible for some 5-10 seconds after the end of totality.  It must certainly have been brighter than Venus.  I have now measured up three separate negatives, and they agree closely in giving a distance from the centre of the Sun of 105.4 minutes, and a position angle of 230°; however, there is some possibility of systematic error, and I have written to the Air Commodore to ask for further details.  If systematic errors can be eliminated, the place should, I think, be useful for orbit determination; it is a week earlier than any other place.”

Thus writes British astronomer Robert d’Escourt Atkinson (1898-1982) about comet C/1948 V1, the “Eclipse Comet of 1948” seen at Nairobi and Mombasa, Kenya on 1 Nov 1948.  It was next observed in the morning sky on 8 Nov 1948, and continued to be followed until 3 Apr 1949.

According to Edward S. Holden (1846-1914), John Martin Schaeberle (1853-1924) discovered a comet-like object on photographic plates taken during the 16 Apr 1893 total solar eclipse, but it has since been determined (Cliver 1989) that this was a coronal mass ejection (CME).

German-born British physicist Arthur Schuster (1851-1934) recorded a comet on photographic plates of the total solar eclipse of 17 May 1882 in Egypt.  The comet moved noticeably during the 1m50s of totality.  It is thought that this comet was a member of the Kreutz sungrazer group of comets.  It has received the designation of X/1882 K1.  The “X/” indicates that there were not enough observations of this comet to determine an orbit.  In fact, the only observations of this comet were during the total solar eclipse.  The comet is sometime called Comet Tewfik—named after the ruler of Egypt at that time in recognition of his hospitality towards the eclipse party.

A comet was discovered during the eclipse of 19 Jul 418 at Constantinople (Istanbul, Turkey) and was observed for four months afterwards.

Seneca the Younger (c. 4 BC – AD 65) writes in his Naturales quaestiones (Natural Questions):

Posidonius, in fact, tells us that during an eclipse of the Sun a comet once appeared which the sun’s proximity had hitherto concealed.

Did Posidonius (c. 135 BC – 51 BC) see this comet, or was he referring to an even earlier observation made by someone else?  With so much of the knowledge of the ancient world lost or destroyed by barbarians and zealots, we may never know.

Clarke, J. 1910, Physical science in the time of Nero; being a translation of    the Quaestiones naturales of Seneca
Cliver, E. W. 1989, Solar Physics, 122:2, 319-333
Federer, C. A. Jr., Sky & Telescope, January 1949, pp. 59-60
Federer, C. A. Jr., Sky & Telescope, March 1949, p. 110, 113
Hetherington, B. 1996, A Chronicle of Pre-Telescopic Astronomy
Kronk, G. W., Cometography, X/1882 K1 (Eclipse Comet or “Tewfik”)
Poitevin, P., Eclipse Comets
Seneca c. 65 AD, Naturales quaestiones, 7.20.4
Vaquero, J. M. 2014, Physics Today, 67:5, 9

Electronic Music

If you haven’t experienced any of the wonderful music courses taught by Dr. Robert Greenberg, available through The Great Courses, you’re missing a lot.  In episode 1, “The Language of Music”, of Understanding the Fundamentals of Music (Course No. 7261), Greenberg describes music not only as a language, but as what I would call a superlanguage.

Music is the ultimate language, a mega-language.  A language in which our hard-wired proclivities to use successions of pitches and sounds to communicate are exaggerated, intensified, and codified into a sonic experience capable of infinitely more expressive depth and nuance than mere words alone.

Greenberg goes on to present a definition of music that is far better than any you will find in the dictionary.

Music is sound in time, or, if you prefer, time ordered by sound.  That definition isolates the two essential aspects of music, sound and time, without any qualifications.

After defining timbre, Greenberg presents the five families of instruments in the Western musical tradition.  Aside from the human voice, they are

  1. Stringed instruments
  2. Wind instruments
  3. Brass instruments
  4. Percussion instruments
  5. Keyboard instruments

And, Greenberg states,

If this course had been written back in the 1970s or ’80s, it would have included a sixth instrumental category: electronics.  There was a genuine belief back then that digitally synthesized sound was the wave of the future.  And that an entirely new vocabulary of sound, one relevant to the technocracy of the modern world, was just around the corner.  You know what?  It never happened.  As it turned out, composers prefer to write for real people playing real instruments.  And audiences would rather listen to real people playing real instruments.  Ironically, more than anything else, digital electronics are used today to imitate those “antiquated” instruments that they were purportedly going to replace.

Though I certainly agree that electronic music will never replace natural instruments played by real people, and I hope that orchestral and chamber music will be with us centuries hence, I have no doubt that new instruments will occasionally be invented and join their venerated ranks, and that electronic music will one day garner enough respect that it will take a permanent seat as a sixth instrumental category.

The world has yet to see a composer of electronic music that can be considered on equal footing with Bach, Beethoven, Brahms, Mozart, or Mahler.  But it will happen.  Perhaps, even today, there lives a young girl or boy somewhere in the world who is already on the path towards becoming the world’s first great composer of electronic music.

Isao Tomita (1932-2016), of Japan, has arguably come the closest.  Yes, his music is idiosyncratic, and his best work a reinterpretation of existing orchestral pieces, but when you listen to Tomita at his best, you get at least a sense of what is possible within the electronic idiom.  Who wouldn’t be tempted by the ability to create any tone color or instrumental timbre imaginable?  It’s not for everyone, I know.

Here is a sampling of Tomita’s best work:

Snowflakes are Dancing (1974)

Pictures at an Exhibition (1975)

Firebird (1976)

Tomita was a pioneer.  The best is yet to come.

Windows to the Earliest: Neutrinos and Gravitational Waves

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

Thesis B7…
Neutrinos and gravitational waves will in principle allow us to peer back to much earlier times (the time of neutrino decoupling and the quantum gravity era respectively), but are much harder to observe at all, let alone in useful directional detail.  Nevertheless the latter has the potential to open up to us access to eras quite unobservable in any other way.  Maybe they will give us unexpected information on processes in the very early universe which would count as new features of physical cosmology.

The cosmic microwave background (CMB, T = 2.73 K) points us to a time 380,000 years after the Big Bang when the average temperature of the universe was around 3000 K.  But there must also exist abundant low-energy neutrinos (cosmic neutrino background, CNB, CνB, relic neutrinos) that provide a window to our universe just one second after the Big Bang during the radiation dominated era.  That’s when neutrinos decoupled from normal baryonic matter.

As the universe expanded, these relic neutrinos cooled from a temperature of 1010 K down to about 1.95 K in our present era, but such low-energy neutrinos almost never interact with normal matter.  Even though the density of these relic neutrinos should be at least 340 neutrinos per cm3 (including 56 electron neutrinos per cm3 which will presumably be easier to detect), detecting them at all will be exceedingly difficult.

Neutrinos interact with matter only through the weak nuclear force (which has a very short range), and low-energy neutrinos are much more difficult to detect than higher-energy neutrinos—if they can be detected at all.  If neutrinos have mass, then they will also interact gravitationally with other particles having mass, but this interaction is no doubt unmeasurable due to the neutrino’s tiny mass and the weakness of the gravitational force between subatomic particles.

The cosmic gravitational background (CGB) points us to the time of the Big Bang itself.  Faessler, et al. (2016) state

The inflationary expansion of the Universe by about a factor 1026 between roughly 10-35 to 10-33 seconds after the BB couples according to General Relativity to gravitational waves, which decouple after this time and their fluctuations are the seed for Galaxy Clusters and even Galaxies. These decoupled gravitational waves run since then with only very minor distortions through the Universe and contain a memory to the BB.

Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.

Faessler, A., Hodák, R., Kovalenko, S., and Šimkovic, F. 2016

Saturn at Opposition: Radiant Rings

If you look at Saturn through a telescope this Thursday morning, you’ll notice something very special about the view.  Saturn’s spectacular ring system—which is presently tilted towards us near its maximum amount—will look unusually bright and white compared with the ball of the planet.  Even though you may have looked at Saturn numerous times before and noticed that the rings are brighter and whiter than the disk of the planet (as well they should be, being composed almost entirely of water ice), you will be wholly unprepared for the view that awaits you this Thursday morning.  The rings will be positively radiant!  Why haven’t you noticed this before?

Saturn, you see, reaches opposition at 5:18 a.m. CDT on Thursday, June 15.  At the moment of opposition in Iowa County, Wisconsin, Saturn will only be 2° above the SW horizon and the Sun just three minutes before sunrise in the NE.  Best to look around 3:03 a.m. at the beginning of astronomical twilight when Saturn will be a respectable 19° above the SSW horizon.

When a superior planet (like Saturn) is at opposition, the Sun, the Earth, and the planet (in that order) form very nearly a straight line.  When we look at Saturn when it is at opposition, we see sunlight reflected off of the icy ring particles pretty much along the path the sunlight took.  Put another way, when light is shining normal (perpendicular) to a reflective surface, more light is reflected back along the normal than is scattered in other directions, so the reflection seems bright.  What’s more, since our line of sight to Saturn’s rings at opposition most closely aligns with the line of incident sunlight, we “see” no shadows from the ring particles, and the rings appear even brighter because of that.  Of course, we can’t see individual ring particles, but when sunlight strikes the rings more from the side, innumerable tiny shadows are cast by ring particles on other ring particles, and the total amount of light reflected back to us is diminished.  This phenomenon is called the opposition effect.

Saturn’s equator and ring plane are tilted 26.7° relative to its orbit around the Sun, so that means we see the rings at different angles throughout Saturn’s 29.5 year orbital period.  For example, the rings were seen edge-on in 1995.  They were tipped 26.7° to the north (meaning we had the best view of the south side of the rings and the southern hemisphere of the planet) in 2003.  The rings were again seen edge-on in 2009, and this year they are tipped 26.7° to the south, meaning we presently have the best view of the north side of the rings and the northern hemisphere of the planet.

Saturn goes through seasons just like the Earth, and the tilt of the rings reflects this.  Just as the northern hemisphere of the Earth is tilted most towards the Sun at summer solstice, Saturn’s northern hemisphere and the north side of its rings are tilted most towards the Sun when Saturn is at its summer solstice.  At the autumnal equinox on Earth, the Earth’s equator lines up with the Sun, and when Saturn is at its autumnal equinox, Saturn’s equator and its rings line up with the Sun.  At winter solstice, the southern hemisphere of the Earth is tilted most towards the Sun.  Likewise, when Saturn is at winter solstice, the south side of Saturn’s rings and its southern hemisphere are tilted most towards the Sun.  At the vernal equinox on Earth, the Earth’s equator once again lines up with the Sun.  And, when Saturn reaches its vernal equinox, Saturn’s equator and its rings once again line up with the Sun.  And so on.  The only difference is our four seasons take a year, but the four seasons on Saturn take nearly 30 years.

The tilt of Saturn’s rings would thus progress peacefully and sinusoidally with two maxima and two minima every 29.5 years—if we were observing from the Sun.  But, of course, we are observing Saturn from spaceship Earth, so the tilt of the rings that we see changes due to our position relative to Saturn in addition to Saturn’s position relative to the Sun.  This causes more rapid—albeit smaller—variations in the observed tilt angle as we orbit much faster around the Sun than Saturn does.  For example, right now we see Saturn’s rings tilted 26.6°.  On July 15, it will be 26.7°.  On August 15, 26.8°.  On September 15, 26.9°.  On October 15, 27.0°.  On November 15, back to 26.9° again.

The best time, then, to see Saturn’s rings at their most radiant is when Saturn is at opposition and when Saturn’s rings are near maximum tilt. Here on Spaceship Earth, that will next occur on Thursday, June 15, 2017.  Fortunately, Saturn’s phase angle will be just 0.1° during all of Wednesday night and Thursday morning.  Saturn rises at 8:27 p.m. Wednesday evening, June 14, and reaches 10° above the SE horizon at 9:40 p.m.  Enjoy!

Habitable Zones

One common definition of the habitable zone of a star is the range of distances from the star where liquid water could exist on the surface of a planet (where the planetary surface temperature ranges between 0° and 100° C [273.15 – 373.15 K]).

Of course, atmospheric pressure affects the temperature range for liquid water.  For example, at 3% of sea level atmospheric pressure, water boils at 26.4° C, not 100° C.  But at 68 atmospheres, water stays liquid until it reaches a scalding temperature of 285° C.  At the other end of the liquid water spectrum of temperatures, the freezing point of water only increases to 0.01° C from 1 atm all the way down to 0.006 atm.  At atmospheric pressures below 0.006 atm, liquid water can’t exist: the only phases that can be present are solid and gas.  At higher pressures, all the way up to about 99 atm, the freezing point of water remains at 0° C.  Then, from 99 atm up to 2,072 atm, the freezing point of water lowers to -21.9° C.  Then it goes back up to 0° C again at 6,241 atm.  Above 70,000 atm, H2O can exist only in solid form.

So, the range of temperature where liquid water can exist is generally smaller at lower atmospheric pressure, and greater at higher atmospheric pressure.

Substances dissolved in the water, called solutes, can also change the range of temperatures where liquid water can exist.  And, who’s to say that life couldn’t exist with only water ice or water vapor in the environment?

And what about life beneath the surface of a planet, moon, asteroid, comet, etc.?  It seems reasonable to suggest that subsurface liquid water exists on more worlds than liquid water on the surface.

And does life always require H2O to exist?

Determining the “habitable zone” of a star is complicated.  That’s why we often narrow it down to just where terrestrial life could exist.

So, for now, let’s stick with that.

As you might expect, many factors enter into the equation: some relate to the star (e.g. size and surface temperature and hence bolometric luminosity), and some relate to the planet (e.g. atmospheric composition & density, and albedo).  A liberal definition might say that the habitable zone in our solar system lies between the orbits of Venus (0.7 AU) and Mars (1.5 AU).

If one accepts this, then the calculation of the habitable zone around any other star is straightforward:


R1 is the inner radius of the habitable zone, in astronomical units
R2 is the outer radius of the habitable zone, in astronomical units
r* is the radius of the star, in solar radii
t* is the effective temperature of the star’s photosphere, in Kelvin

Here’s an example that’s made big news lately: seven planets very similar in size to the Earth have been discovered orbiting the red dwarf star TRAPPIST-1, located 39 light years from our solar system in the direction of the constellation Aquarius.  The estimated size of the star is 0.117 solar radii, and the estimated effective temperature 2559 K.  Using the above equations, we get R1 = 0.016 AU and R2 = 0.034 AU. Thus, using our approach, it appears that planets TRAPPIST-1d (0.772 R) and TRAPPIST-1e (0.918 R) are most likely to be within the star’s habitable zone.

The Sachs-Wolfe Effect

The cosmic microwave background (CMB) peaks at a wavelength of 1.9 mm and frequency 160.23 GHz, if spectral radiance is defined in terms  of frequency.  If spectral radiance is defined in terms of wavelength, then the CMB peaks at wavelength 1.1 mm.  This radiation comes from all directions, and the curve of intensity as a function of wavelength very closely approximates a perfect black body having temperature 2.725 Kelvin.  Since the Big Bang 13.8 billion years ago, the universe has expanded and cooled so that today its temperature is 2.725 K.

About 380,000 years after the Big Bang, the universe had expanded and cooled enough so that for the first time it became transparent to electromagnetic radiation.  Thus when we accurately map the exact spectrum of the cosmic microwave background in different directions, we can construct a “baby picture” of the universe when it was only 380,000 years old.

Our baby picture is not smooth but has features.  At that early time, the universe had already developed into denser regions, and less dense ones. Now, it is important to note that cosmic microwave background photons that left a denser part of the universe have been gravitationally redshifted to slightly longer wavelengths (and lower frequencies) to a greater extent than elsewhere.  This is called the Sachs-Wolfe effect.


Lots of exciting cosmological information is coming out of mapping the tiny differences in the CMB spectrum as we look in different directions. I’m wondering, though, if anyone has seen temporal variations in the CMB? In other words, if you stay pointed in a particular direction and carefully measure the CMB spectrum over time, does it change or fluctuate at all (after all sources of noise have been removed)?  Even though our current understanding of cosmology might lead us to believe that the CMB would not change fast enough for us to measure, has anybody looked?

An Open Letter to an Unknown Neighbor

We haven’t met yet.  I’m a non-confrontational kind of person (a typical Midwestern trait, I’ve heard), always eager to please and not to offend.  But I want you to know how much your dusk-to-dawn floodlight bothers me.  You see, I’m an astronomer.  I even have a backyard observatory and I would love to show you the wonders of the night sky if you’re interested in seeing what’s up there.  I’m probably the only person in Dodgeville or Iowa County doing astronomical research several nights a week, weather permitting.  I accurately time when asteroids and trans-Neptunian objects pass in front of stars, blocking their light for fractions of a second up to several seconds.  There is a lot we can learn from such events.

When I moved into my house, I had to install thick curtains in my bedroom because your bright light floods into the room all night long every night.  In fact, your light floods into every window on the west side of my house.

I like it dark at night.  It helps me to sleep better and, I’ve heard, sleeping darker is sleeping healthier.  There’s even medical research that supports this.

Being an astronomer, I like to step outside and check the night sky from time to time, look at constellations—see if the northern lights are active.  All of this is a struggle for me now.  But it doesn’t need to be.

I think I know why you want to have this light.  It seems you are trying to light the stairway from your backyard to your front yard for safety reasons when using those stairs at night.  Have you considered putting those floodlights on motion sensors instead of a dusk-to-dawn timer?  You’d save money on bulbs and electricity.  Or, if you really feel you need the light to be on all night long, a better lighting system could be installed that would light your stairs without lighting up your neighbors’ houses and yards.  Can’t afford it?  I’m not wealthy either, but I’d be more than willing to pay for the lighting improvements, because I want to be a good neighbor and having a dark backyard and house at night means that much to me.  Besides, one of the benefits of living in a small town in this beautiful area of rural southwest Wisconsin is getting a decent view of the night sky.  No big city can compete with that.

I’ll even pay for us to hire a professional lighting engineer to do the job right so both you and I (and probably your other neighbors) will be thrilled with the results.  I know enough about lighting to say confidently we will have a win-win situation.  Guaranteed.

I’m looking forward to meeting you and discussing this.  Thank you.

Small Universe

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

4.3.1 Small universes
A Small Universe: a universe which closes up on itself spatially for topological reasons, and does so on such a small scale that we have seen right round the universe since the time of decoupling.  Then we can see all the matter that exists, with multiple images of many objects occurring.  This possibility is observationally testable by examining source statistics, by observation of low power in the large angle CBR anisotropies, and by detecting identical temperature variation on various circles in the CBR sky.  There are weak hints in the observed CBR anisotropies (the lack of power on large angular scales) that this could actually be the case, but this is not solidly confirmed.  Checking if the universe is a small universe or not is an important task; the nature of our observational relationship to the universe is fundamentally different if it is true.

In 1900, Karl Schwarzschild (1873-1916) was perhaps the first to suggest the idea of a small universe topology that would lead to multiple images of the same object at different points in the past.  Though most cosmologists favor the idea of a very large universe with a simple topology, the possibility of a more complex small universe topology is still not out of the question.  The universe might be measurably finite in some or all directions.

The smaller a finite topological region of space, the easier it should be to discover multiple images of the same object at different ages (except for CMB features which will all be the same age).  The distribution of distant sources might show “patterns” that are related to more nearby sources.  A comprehensive survey of sources at redshifts between about z=2 to z=6 is still needed before any conclusions can be drawn.

Another approach, of course, is to look at patterns in the CMB temperature (intensity) and polarization.  Analyses of the most recent release of Planck satellite data, however, shows no evidence of a compact topology smaller than our visual horizon.

Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.

Luminet, J.-P. 2016,  arXiv:1601.03884v2 [astro-ph.CO]