News from LISA

[Nomad]

Ευχάριστα νέα από το μέτωπο της βαρύτητας [1] και το μεγάλο πείραμα ανίχνευσης βαρυτικών κυμάτων LISA (LASER Interferometer Space Antenna) [2,3].

Βρέθηκε τρόπος να διατρηθεί σε ευθυγράμμιση η διάταξη των LISA probes (τριγωνική διάταξη με απόσταση 5Gm με το κέντρο μάζας σε τροχιά πολύ κοντά σε αυτή της Γης, με καθυστέρηση 20 μοιρών [2]). Το πρόβλημα είναι ότι πέρα από ένα σφάλμα (υποατομικής σχεδόν κλίμακας, [1]) στο προσανατολισμό των κατόπτρων μπορεί να δώσει εσφαλέμνο σήμα. [1] Έχουν προταθεί δύο ευαίσθητες μεθόδοι ρύθμισης και ελέγχου, για λεπτομέρειες βλ. [1].

Το LISA pathfinder θα εκτοξευτεί πριν το LISA deployment για να τεστάρει key τεχνολογίες σαν τη παρούσα στις συνθήκες σχεδόν του πειράματος. [1]

[1]: http://www.esa.int/SPECIALS/Space_Engin ... BKG_0.html
[2]: http://www.esa.int/esaSC/120376_index_0_m.html
[3]: http://sci.esa.int/science-e/www/area/i ... fareaid=27



Each probe has two arms spanning in 60 degrees (so that the three of them form an equilateral triangle), each containing a test mass in free fall.


Inside the two cubes are the test masses, that will be placed in each of the LISA probes (one on each arm). The principle of measurement in this large interferometry experiment is that the test masses are kept in free fall.

[Nomad]

Κριμα που πρέπει να περιμένουμε μέχρι το 2013.

To 2013 είναι το pathfinder αν κατάλαβα καλά... Για το LISA θα περιμένουμε μέχρι το 2020!

Η ιστορία πάντως με τα βαρυτικά κύματα πάει πολύ πίσω στο χρόνο και εδώ και δεκαετίες γίνονται προσπάθιες με LASER interferometers (4km, LIGO type detectors (πιο παλιά υπήρχαν και οι mass detectors που ξεπεράστηκαν (το CERN είχε τον explorer nομίζω))). Μιλάμε για παραμορφώσεις της τάξεως του που πρέπει να ανιχνευτούν (εξού και οι τεράστιες αποστάσεις (5Gm για το LISA) για να απαλύνουν λίγο αύτο το ποσοστό (χοντρικά)). Γι' αυτό και έχει τόση σημασία η πολύ λεπτή ρύθμιση των κατόπτρων (πέρα από την ευαισθησία ευθυγράμμισης σε αυτή τη τερατώδη απόσταση!)

[Nomad]

Είναι αυτό που λέει η λέξη. Λαμβάνοντας υπόψη τη γενική θεωρία της σχετικότητας, η οποία χοντρικά μιλάει για την έκφανση της βαρύτητας ως καμπύλωση του χωροχρόνου, παρουσία μάζας, τα βαρυτικά κύματα είναι ripples in spacetime, "περιοδικές διαταραχές του χωροχρόνου" αν θέλεις. Ως εκ τούτου πηγές βαρυτικών κυμάτων μπορούν* να είναι μελάγα αστρικά σώματα με δεδομένες ασυμμετρίες. [1]

Οι μέθοδοι ανίχνευσης που έχουν αναπτυχθεί είναι βασικά 2:

• Οι mass detectors [2 (Weber bars)]
  Μέτρηση τυχών παραμορφώσεων πάνω σε ένα σώμα. (με πιεζοκρυστάλλους ίσως?) Δε ξέρω, μπορεί να λέω και μπούρδες. Η αλήθεια είναι ότι δεν υπάρχει ελπίδα για αυτή τη μέθοδο, μπροστά στην
• Laser Interferometry [2 (Interferometers)]
  Η διατάξεις ποικίλουν, η αρχή όμως είναι η μέτρηση μιας (ή δύο μάλλόν) καθυστέρηση φάσης σε πείραμα συμβολής που ενδεχομένως προκαλείται από την εν τω μεταξύ (χρονικά) παραμορφωση του χρόνου (χοντρά το λεω) η οποία καθορίζεται από τη συχνότητα των βαρυτικών κυμάτων που και αυτά διαδίδονται με τη ταχύτητα του φωτός, άρα μεταφράζοντας σε μήκος κύματος, το μέγεθος του ανιχνευτή καθορίζει και το κομμάτι του φάσματος που θεωρητικά θα μπορεί να ανιχνέυσει. (βλ. εικόνα)

Οι LIGO detectors που είναι 4km ενδέχεται να ανιχνεύσουν από τις πιο υψηλές συχνότητες (HF, που σε σχέση με τα ηλεκτρομαγνητικά κύματα είναι πολύ χαμηλές, microwave σχεδόν). Ακόμα δεν έχει κατορθωθεί μέσα σε τρεις δεκαετίες LIGO νομίζω αλλα υπάρχει ένα enhancement program που τρέχει αυτή τη δεκαετία προς αυτή τη κατεύθυνση.

Το LISA με τα 5Gm του στο διάσημα, απαλλαγμένο απο "γήινο θόρυβο" αλλα με άλλα προβλήματα όπως το calibration (που είναι και το θέμα αυτού του topic), θα είναι πιο ευαίσθητο σε ποιο χαμηλές συχνότητες (LF) ενώ οι πολύ χαμηλές (VLF και ULF) είναι απρόσιτες (even conceptually**). Λέγεται ότι τα ULF gravitational waves έχουν μήκος κύματος συγκρίσιμο με τις διαστάσεις του σύμαντος! (και ότι η δημιουργία τους τοποθετείται ως εκ τούτου κοντά, στο big bang, εικασία, και μπορεί και να μη το λέω καλά...)

*δεδομένης της υπόθεσης ότι δεν έχουν ανιχνευτεί ακόμα...
**δεν είναι τόσο αλήθεια, γιατί όπως βλέπεις και από την εικόνα, εικάζονται μέθοδοι μέτρησης τους (pulsar timing για ULF, shift in polarization of Cosmic Microwave Background για ULF) αλλά είναι σε πολύ πρώιμο στάδιο πιστεύω. (χρειάζεται επιβεβαίωση για το pulsar timing)

[1]: http://en.wikipedia.org/wiki/Gravitational_Waves
[2]: http://en.wikipedia.org/wiki/Gravitatio ... e_detector

[fakiris]

@Nomad:

Οι mass detectors η αλήθεια είναι ότι έχουν πειραματικά αρχίσει να παραγκωνίζονται. Ίσως λόγω του ότι δεν μπορούν να αναπαραστήσουν με ικανοποιητικά επιχειρήματα την ισχύ των αποτελεσμάτων ανίχνευσης. Πραγματικά, όμως έχει προχωρήσει αρκετά η δεύτερη μέθοδος. Αρκετά καλό.

[barbara]

Το pathfinder (LPF) είναι όντως να εκτοξευθεί το 2013. Υπάρχουν όμως καθυστερήσεις στις παραλλαβές των κομματιών του από τα συνεργαζόμενα ιδρύματα, άρα μπορεί να πάει και το '14 ή ακόμα και το '15. Αυτή τη στιγμή βρίσκεται στην φάση της συναρμολόγησης (στην Γερμανία νομίζω), ενώ γίνονται και οι τελευταίοι έλεγχοι στα on-board ηλεκτρονικά.

[HeartDoc]

Off Topic

[quote="barbara"]Το pathfinder (LPF) είναι όντως να εκτοξευθεί το 2013. Υπάρχουν όμως καθυστερήσεις στις παραλλαβές των κομματιών του από τα συνεργαζόμενα ιδρύματα, άρα μπορεί να πάει και το '14 ή ακόμα και το '15. Αυτή τη στιγμή βρίσκεται στην φάση της συναρμολόγησης (στην Γερμανία νομίζω), ενώ γίνονται και οι τελευταίοι έλεγχοι στα on-board ηλεκτρονικά.[/quote]

Άμα έχουμε τους experts να απαντάνε.! Vamos senior!

[Nomad]

Off Topic

Άμα έχουμε τους experts να απαντάνε.! Vamos senior!

Off Topic

Μα για να τσιμπήσει o Νίκος, postάρα το topic.~

[barbara]

Off Topic

Άμα έχουμε τους experts να απαντάνε.! Vamos senior!

Off Topic

Μα για να τσιμπήσει o Νίκος, postάρα το topic.~

Off Topic

χεχε! τι καθάρματα που έιστε! θα γρ´αψω κ άλλα όταν πάρω το κολάι!

[barbara]

European Space Agency to go it alone on next generation space mission - March 17, 2011

The European Space Agency has postponed its selection of a large space mission to launch in the 2020-2021 timeframe, following advice from NASA that the US is unlikely to be able to contribute its share of funding to the winning selection. The selection of the so-called L-class mission had been expected to take place this June, but is now set for February 2012. “The decision was made very reluctantly,” says David Southwood, Director of Science and Robotic Exploration at ESA Headquarters in Paris, France,“NASA could not meet our timetable to launch.”

The three competing missions affected by the decision (pictured from top right to bottom right) are the Laser Interferometer Space Antenna (LISA), which would detect gravitational waves from space, the Europa-Jupiter System Mission, a mission to the moons of Jupiter (called ESJM-Laplace), and the International X-ray Observatory (IXO), an X-ray space telescope. In a 14 March email, Fabio Favata, ESA coordinator for astronomy and fundamental physics missions asks member of ESA’s Science Program Committee, which will choose between the three proposed missions, to work out whether they could fly on a Europe-only budget , with minimal cooperation from NASA. Under such a scheme, says Southwood, the 700 million Euros that ESA has budgeted for the L-Class mission must now be considered as the mission's total cost rather than 40-50% of a joint NASA-ESA effort. “It’s a big change and maybe the challenge can’t be met,” Southwood acknowledges.

NASA’s reluctance to commit to ESA’s timetable was a consequence of the tight fiscal situation in the US federal government, and two recent US advisory reports that ranked the L-class missions below others as the top US funding priorities. In August 2010, the Astro2010 decadal survey ranked IXO and LISA below the Wide Field Infrared Survey Telescope (WFIRST) a planned infrared space telescope that would take measurements to get a handle on the nature of the dark energy pervading the universe. On March 7, NASA’s planetary sciences decadal survey ranked the mission to Jupiter below a possible sample return mission from Mars. In the ten days since the planetary sciences decadal survey Southwood and his counterparts at NASA have been analyzing the situation and concluded that NASA would now be unlikely to to be a major partner in any of the missions – whichever one was selected – on the timescale that ESA needed for the mission to make the launch date. Rather than give up on the idea of a larger space mission, ESA has decided to go it alone, and has now asked the mission scientists to rewrite their proposals accordingly, necessitating the delay, Southwood says.

The next step will be for the teams behind LISA, IXO and ESJM-Laplace to rescope their missions for the rejigged competition. Astrophysicist Jack Hughes at Rutgers University in New Brunswick, New Jersey, a member of the science definition team for IXO, says he’s cautiously optimistic. It’s already been clear from NASA’s difficulties with the cost overrun of the James Webb Space Telescope (now estimated at $6.5 billion) that the agency would probably not be able to invest in a next generation space mission until JWST has launched, so it’s good to see that ESA’s prepared to move forward without it, he says. “ESA’s being practical and saying what can we do to advance the science goals we’re interested in and I think that’s a rational and logical approach,” he says.

In a related story, the blog SpacePolicyOnline.Com notes that NASA must now depend on a renewed partnership with ESA to fly the MAX-C sample return mission to Mars -- the highest rated project in the planetary decadal survery -- because it cannot afford the $2.5 billion needed to underwrite the mission's intial stage. The revelation was made yesterday by Jim Green, director of NASA's Planetary Science Division during a meeting of the Planetary Science Subcommittee of the NASA Advisory Council. NASA and ESA officials are scheduled to meet at the end of this month to discuss the agencies' planetary partnerships and implications for future joint missions.

πηγή: http://blogs.nature.com/news/thegreatbeyond/

[apolski]

Did the early universe have just one spatial dimension?

That's the mind-boggling concept at the heart of a theory that University at Buffalo physicist Dejan Stojkovic and colleagues proposed in 2010.

They suggested that the early universe -- which exploded from a single point and was very, very small at first -- was one-dimensional (like a straight line) before expanding to include two dimensions (like a plane) and then three (like the world in which we live today).

The theory, if valid, would address important problems in particle physics.

Now, in a new paper in Physical Review Letters, Stojkovic and Loyola Marymount University physicist Jonas Mureika describe a test that could prove or disprove the "vanishing dimensions" hypothesis.

Because it takes time for light and other waves to travel to Earth, telescopes peering out into space can, essentially, look back into time as they probe the universe's outer reaches.

Gravitational waves can't exist in one- or two-dimensional space. So Stojkovic and Mureika have reasoned that the Laser Interferometer Space Antenna (LISA), a planned international gravitational observatory, should not detect any gravitational waves emanating from the lower-dimensional epochs of the early universe.

http://www.sciencedaily.com/releases/20 ... 152059.htm

[barbara]

LISA Pathfinder takes major step in hunt for gravity waves

14 November 2011
Sensors destined for ESA’s LISA Pathfinder mission in 2014 have far exceeded expectations, paving the way for a mission to detect one of the most elusive forces permeating through space – gravity waves.

The Optical Metrology Subsystem underwent its first full tests under space-like temperature and vacuum conditions using an almost complete version of the spacecraft.

The results exceeded the precision required to detect the enigmatic ripples in the fabric of space and time predicted by Albert Einstein – and did it by two to three times.

In space, LISA Pathfinder will measure the distance between two free-floating gold–platinum cubes using lasers. In the ground tests currently being performed by the team in Ottobrunn, Germany, these cubes are replaced by separate mirrors.

In addition to measuring the distance between the cubes, it also measures their angles with respect to the laser beams – and the tests show an accuracy of 10 billionths of a degree.

“This is equivalent to the angle subtended by an astronaut’s footprint on the Moon!” notes Paul McNamara, Project Scientist for the LISA Pathfinder mission.

Under perfect conditions in space, the free-floating cubes would be expected to exactly copy each other’s motions.

However, according to Einstein’s general theory of relativity, if a gravitational wave were to pass through space, possibly caused by an event as catastrophic as the collision of two black holes, then a minuscule distortion in the fabric of space itself would be detectable.

The accuracy required to detect such a subtle change is phenomenal: around a hundredth the size of an atom – a picometre.

The requirement set for the instrument was around 6 picometres, measured over 1000 seconds, which the team initially bettered in 2010.

During the latest testing, a staggering 2 picometre accuracy was obtained, far exceeding the best performance for an instrument of this type.

“The whole team has worked extremely hard to make this measurement possible,” said Dr McNamara.

“When LISA Pathfinder is launched and we're in the quiet environment of space some 1.5 million km from Earth, we expect that performance will be even better.”

The instrument team from Astrium GmbH, the Albert Einstein Institute and ESA are testing the Optical Metrology Subsystem during LISA Pathfinder thermal vacuum tests in Ottobrunn by spacecraft prime contractor Astrium (UK) Ltd.

LISA Pathfinder is expected to be launched in mid-2014 to demonstrate the technologies and endurance in space for a New Gravitational wave Observatory mission, one of the candidates for ESA’s next flagship mission, planned for a launch early in the next decade, aiming to find this final piece in Einstein’s cosmic puzzle.

Πτήση τον ιούνη 2014! ΓΟΥΤ!

source: http://www.esa.int/esaSC/SEMX61WWVUG_index_0.html

[Nomad]

One small step for LISA... One giant leap for gravity...

[barbara]

The hot and energetic Universe and the search for elusive gravitational waves will be the focus of ESA’s next two large science missions, it was announced today.

Both topics will bridge fundamental astrophysics and cosmology themes by studying in detail the processes that are crucial to the large-scale evolution of the Universe and its underlying physics.

The science theme “the hot and energetic Universe” was selected for L2 – the second Large-class mission in ESA’s Cosmic Vision science programme – and is expected to be pursued with an advanced X-ray observatory.

This mission, with a launch date foreseen for 2028, will address two key questions. How and why does ordinary matter assemble into the galaxies and galactic clusters that we see today, and how do black holes grow and influence their surroundings?

Black holes, which lurk unseen at the centres of almost all galaxies, are regarded as one of the keys to understanding galaxy formation and evolution.

The L3 mission will study the gravitational Universe, searching for ripples in the very fabric of space–time created by celestial objects with very strong gravity, such as pairs of merging black holes.

Predicted by Einstein’s theory of general relativity but yet to be detected directly, gravitational waves promise to open a completely new window on the Universe.

Planned for launch in 2034, it will require the development of a spaceborne gravitational wave observatory, or extreme precision ‘gravitometer’, an ambitious enterprise that will push the boundaries of current technology.

“ESA has an outstanding record for developing state-of-the art space observatories that have revolutionised our knowledge of how stars and galaxies were born and evolved,” says Alvaro Gimenez, ESA’s Director of Science and Robotic Exploration.

“By pursuing these two new themes, we will continue to push back the scientific boundaries and unveil the mysteries of the invisible Universe.”

The selection process for L2 and L3 began in March 2013, when ESA issued a call to the European science community to suggest the next scientific themes that should be pursued by the Cosmic Vision programme’s Large missions.

Thirty-two proposals were received and assessed by a Senior Survey Committee, and following an extensive interaction with the scientific community two major themes were recommended to the Director of Science and Robotic Exploration.

“We had a difficult task in deciding which scientific themes to choose from all of the excellent candidates, but we believe that missions to study the hot, energetic Universe and gravitational waves will result in discoveries of the greatest importance to cosmology, astrophysics, and physics in general,” says Catherine Cesarsky, chair of the Senior Survey Committee.

Although the launch dates for the L2 and L3 missions are more than a decade away, activities to prepare the missions will start very soon. Early in 2014, a call for L2 mission concepts will be announced to solicit proposals for a next-generation X-ray observatory. A similar procedure will be followed at a later date for the L3 mission.

“We have opened up a new scientific roadmap for Europe today that will establish our leadership in this field for the next two decades while we develop and implement new technologies for these exciting missions,” adds Prof. Gimenez.

source: http://www.esa.int/Our_Activities/Space ... e_Universe

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